REVIEW
published: 07 March 2017
doi: 10.3389/fncel.2017.00054
The GABAergic Hypothesis for
Cognitive Disabilities in Down
Syndrome
Andrea Contestabile 1 *, Salvatore Magara 1 and Laura Cancedda 1,2 *
1
Department of Neuroscience and Brain Technologies, Istituto Italiano di Tecnologia (IIT), Genova, Italy, 2 Dulbecco Telethon
Institute, Genova, Italy
Down syndrome (DS) is a genetic disorder caused by the presence of a third copy of
chromosome 21. DS affects multiple organs, but it invariably results in altered brain
development and diverse degrees of intellectual disability. A large body of evidence
has shown that synaptic deficits and memory impairment are largely determined by
altered GABAergic signaling in trisomic mouse models of DS. These alterations arise
during brain development while extending into adulthood, and include genesis of
GABAergic neurons, variation of the inhibitory drive and modifications in the control of
neural-network excitability. Accordingly, different pharmacological interventions targeting
GABAergic signaling have proven promising preclinical approaches to rescue cognitive
impairment in DS mouse models. In this review, we will discuss recent data regarding
the complex scenario of GABAergic dysfunctions in the trisomic brain of DS mice and
patients, and we will evaluate the state of current clinical research targeting GABAergic
signaling in individuals with DS.
Edited by:
Daniela Tropea,
Trinity College, Dublin, Ireland
Reviewed by:
Enrico Cherubini,
Scuola Internazionale di Studi
Superiori Avanzati (SISSA), Italy
Nicola Berretta,
Fondazione Santa Lucia (IRCCS),
Italy
*Correspondence:
Andrea Contestabile
[email protected]
Laura Cancedda
[email protected]
Received: 21 December 2016
Accepted: 14 February 2017
Published: 07 March 2017
Citation:
Contestabile A, Magara S and
Cancedda L (2017) The GABAergic
Hypothesis for Cognitive Disabilities
in Down Syndrome.
Front. Cell. Neurosci. 11:54.
doi: 10.3389/fncel.2017.00054
Keywords: GABA, down syndrome, cognitive impairment, gaba receptors, chloride homeostasis
INTRODUCTION
Down syndrome (DS) or trisomy 21 is the leading cause of genetically-defined intellectual disability
and congenital birth defects. DS is characterized by many phenotypical features affecting almost all
body systems, including developmental defects and growth delay (Nadel, 2003; Antonarakis and
Epstein, 2006). In particular, brains of individuals with DS show decreased volume and reduced
neuronal density in diverse brain areas (e.g., cortex, hippocampus and cerebellum; Sylvester, 1983;
Coyle et al., 1986; Aylward et al., 1997, 1999; Shapiro, 2001). These alterations originate early
during development (Schmidt-Sidor et al., 1990; Winter et al., 2000; Pinter et al., 2001; Larsen
et al., 2008) and are possibly due to defective neuronal precursor proliferation during gestation
(Contestabile et al., 2007; Guidi et al., 2008). Accordingly, the development of individuals with
DS is characterized by delayed cognitive progress in infancy and childhood, leading to mild-tomoderate mental retardation with an Intelligence Quotient (IQ) ranging from 30 to 70 (Vicari
et al., 2000, 2005; Pennington et al., 2003; Vicari, 2004). This scenario is additionally worsened
during adulthood by further loss of cognitive abilities and the development of Alzheimer’s disease
(AD) by the fourth decade of life (Wisniewski et al., 1985; Mann and Esiri, 1989; Leverenz and
Raskind, 1998; Teipel and Hampel, 2006).
Although cognitive impairment is the most common and severe feature of DS, other
neurological and psychiatric manifestations of the disease highly impinge on the quality of life of
individuals with DS and their families. In particular, the DS population shows increased frequency
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The GABAergic Hypothesis in Down Syndrome
cases not sufficient to reach the threshold for action-potential
generation, they will be able to activate voltage-gated calcium
channels (VGCCs), remove the voltage-dependent magnesium
block from NMDA receptors (Leinekugel et al., 1995, 1997; BenAri, 2002), and possibly add up to concurrent excitatory inputs
(Gulledge and Stuart, 2003). Consequently, even an initially
mild depolarizing effect of GABA can contribute to a more
pronounced excitation and eventually reach the threshold for
action potential firing. Thus, GABAA -mediated transmission is
generally depolarizing and possibly excitatory in the immature
brain, although depolarizing GABAA currents may still be
inhibitory by shunting concurrent excitatory inputs. Notably, a
depolarizing (rather than hyperpolarizing) GABAA R response
may also occur in mature neurons depending on timing and
location of GABAA Rs, upon relatively small difference in [Cl− ]i
at different cell compartments, more hyperpolarized resting
membrane potentials, sustained activity or even pathological
conditions (Lamsa and Taira, 2003; Khirug et al., 2008; Chiang
et al., 2012; Cellot and Cherubini, 2014). Indeed, as the reversal
potential for GABAA R-mediated chloride currents (ECl ) sits
very near to the resting membrane potential (V REST ), relatively
small changes in [Cl− ]i are sufficient to change the polarity
of GABAergic responses. In particular, if ECl is more negative
than V REST , the chloride flow will be inward (hyperpolarizing).
Instead, if ECl is less negative than V REST , the chloride flow
will be in the opposite direction, generating depolarizing
PSPs.
GABAA Rs are composed of five subunits, and the many
subunit variants identified in mammals (six α, three β, three
γ, one δ, three ρ, one ε, one π and one θ) confer different
pharmacological and kinetic properties to the receptor (Olsen
and Sieghart, 2008; Fritschy and Panzanelli, 2014). In the
CNS, the largest population of GABAA Rs is composed of 2 α,
2 β and 1 γ subunits, but isoform expression and receptor
composition change with subcellular localization, brain area,
and development, thus assuring receptor properties set to fulfil
specific neuronal needs. For example, the α1–3 , β2–3 and γ2
isoforms are the most represented in the adult brain, constituting
synaptic GABAA Rs. In particular, thanks to their fast kinetics
with rapid onset and desensitization, they generate phasic
currents following GABA release from presynaptic vesicles with
spatial and temporal accuracy. Conversely, α5 - or δ-containing
GABAA Rs are mainly extra-synaptic. The presence of the α5 or
δ subunits confers high GABA affinity and slow desensitization
kinetics to GABAA Rs. Thus, α5 or δ–containing receptors are
able to detect ambient GABA spilled out of the synaptic cleft
and generate long-lasting (tonic/extrasynaptic) currents (Böhme
et al., 2004; Caraiscos et al., 2004; Farrant and Nusser, 2005;
Zheleznova et al., 2009).
Phasic vs. tonic GABAA -mediated currents provide for
different regulation of neuronal physiology. Phasic GABA
transmission plays a relevant role in synaptic-input integration.
Indeed, fast inhibition acts as coincidence detector of excitatory
inputs by feed-forward inhibition: this mechanism ensures
that a second input desynchronized with a first input is
canceled out; thus, excitatory inputs sum up only when
they are perfectly synchronized (Pouille and Scanziani, 2001).
of anxiety (Vicari et al., 2013), clinically relevant sleep
disturbance (Carter et al., 2009; Breslin et al., 2011; Angriman
et al., 2015; Edgin et al., 2015; Konstantinopoulou et al., 2016;
Maris et al., 2016), and hyperactivity or movement disorders
(Pueschel et al., 1991; Haw et al., 1996). Finally, DS patients
demonstrate increased incidence of epileptic episodes with
seizure onset mainly concentrated during early life and aging
(Stafstrom et al., 1991; Lott and Dierssen, 2010; Robertson et al.,
2015).
The identification of possible mechanisms leading to
cognitive impairment in DS has been largely conducted
thanks to the analysis of different DS genetic mouse models
(Dierssen, 2012). Interestingly, compelling studies in these
animals indicate that altered signaling of the neurotransmitter
GABA is one of the main determinants in reducing cognitive
and memory functions. In particular, GABAergic dysfunctions
impair synaptic plasticity and learning and memory in DS
by altering optimal excitatory/inhibitory synaptic balance
(Kleschevnikov et al., 2004, 2012a,b; Costa and Grybko, 2005;
Fernandez et al., 2007; Deidda et al., 2014). In this review article,
we will survey available data on the alteration of GABAergic
signaling in DS trisomic animal models and individuals with DS.
Additionally, since no effective pharmacological treatment for
ameliorating cognitive deficits in DS has been found yet, we will
also evaluate the current status of pre-clinical and clinical trials
with GABAergic drugs in DS.
GABA SIGNALING IN THE BRAIN
GABA is the main inhibitory neurotransmitter in the adult and
healthy brain and it acts through ionotropic and metabotropic
receptors (GABAA Rs and GABAB Rs, respectively).
GABAA Receptors
GABAA Rs are chloride-permeable ion channels. In the adult
brain, the chloride gradient across the neuronal cell membrane
is sustained by the exporter K-Cl cotransporter (KCC2), which
maintains low intracellular chloride concentration ([Cl− ]i ).
Thus, GABAA R opening generates an influx of negative chloride
ions that hyperpolarizes the cell membrane potential and
inhibits neuronal activity. Moreover, opening of GABAA R
ion channels may also shunt concurrent excitatory currents
(e.g., driven by the neurotransmitter glutamate), thus preventing
them from bringing the membrane potential to the action
potential threshold. Indeed, GABAA R opening short-circuits
depolarizing synaptic currents by locally reducing the input
resistance (Ben-Ari, 2002; Jonas and Buzsaki, 2007; Silver,
2010; Khazipov et al., 2015). Thus, adult GABAA -mediated
transmission is physiologically hyperpolarizing and inhibitory.
Conversely, in early neurodevelopment, KCC2 expression is
low, and associated with high expression of the chloride
importer Na-K-Cl cotransporter (NKCC1), which generates
high [Cl− ]i . In this condition, the chloride flux through
GABAA Rs is outward and depolarizes the cell membrane
potential (Cherubini et al., 1990; Rivera et al., 1999; BenAri, 2002; Fiumelli et al., 2005). Although GABAA R-mediated
depolarizing postsynaptic potentials (PSPs) will be in most
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The GABAergic Hypothesis in Down Syndrome
virtually form protein complexes with GABAB R subunits
in heterologous systems (David et al., 2006; Fowler et al.,
2007; Ciruela et al., 2010). Nevertheless, GABAB R-triggered
potassium currents seem to be mostly mediated by GIRK2containing homo- and hetero-tetramers (Koyrakh et al., 2005).
GIRK channel opening elicits an outward hyperpolarizing K+
current and a decrease in the input resistance (Lüscher et al.,
1997; Koyrakh et al., 2005). Thus, through GIRK channels,
GABAB Rs inhibit neuronal excitability by shunting excitatory
currents, hyperpolarizing the membrane with slow inhibitory
PSPs, and contributing in maintaining V REST (Lüscher et al.,
1997; Koyrakh et al., 2005; Gassmann and Bettler, 2012).
These effects also prevent action potential back-propagation
in dendrites (Leung and Peloquin, 2006). Finally, through
inhibition of postsynaptic voltage-gated calcium channels,
GABAB R activation also prevents dendritic calcium spikes
(Chalifoux and Carter, 2011).
A similar although opposite integration occurs when GABA is
depolarizing. GABAA R opening may generate small excitatory
potentials that last after the channel closure and sum up with
concurrent excitatory inputs, thus facilitating the achievement
of action potential threshold (Gulledge and Stuart, 2003).
While phasic inhibition participates in synaptic integration,
tonic inhibition generates relatively persistent changes of
input conductance, thus rightward shifting the input-output
relationship. Therefore, tonic inhibition reduces the magnitude,
duration and propagation length of excitatory PSPs, limiting
their temporal/spatial summation possibilities and inhibiting
neuronal excitability. Such effect is mainly mediated by shunting
which is particularly relevant for the inhibitory action of tonic
GABA transmission and less for the phasic inhibition, given the
longer opening of tonic GABAA Rs (Ben-Ari, 2002; Farrant and
Nusser, 2005; Jonas and Buzsaki, 2007; Silver, 2010; Khazipov
et al., 2015).
Integration processes of excitatory inputs depend from
PSPs generated from GABAA Rs, shunting inhibition, and also
from the localization of GABAA Rs. For example, on the
dendrites of CA1 pyramidal neurons, GABAergic synapses are
mostly located on the dendritic shaft and on spines (Megías
et al., 2001), thus participating to synaptic integration of two
different sets of inputs: the ones already partially processed
along the way from their synaptic origin to the dendritic
shaft, and the ones originated in neighborhood synapses
in the same spine, respectively. Notably, different cellular
compartments show different [Cl− ]i that critically determine
GABAA R participation to input integration in a compartmentspecific fashion. Indeed, [Cl− ]i is maximum in the axon and
axonal initial segment, lower in the soma, and it continues
to decrease gradually along dendrites (Szabadics et al., 2006;
Khirug et al., 2008; Waseem et al., 2010). In summary, phasic
inhibition plays as a coincidence detector and undergoes to
tight spatial and temporal integration with excitatory inputs
in a compartment-specific manner; tonic inhibition sets the
neuronal excitability background, by affecting excitatory inputoutput relation.
GABAA and GABAB Receptor Functions
Across Neurodevelopment and Adulthood
Given the variety of potential signaling dynamics by GABAA
and GABAB receptors, it is not surprising that GABA may
play a relevant role in different processes during healthy
neurodevelopment and adulthood, as well as under pathological
conditions. In particular, depolarizing and possibly excitatory
GABAA -mediated transmission in early life plays key roles in
promoting neurodevelopment. α5 - or δ-containing extrasynaptic
GABAA Rs are predominant at this stage when synapses are
not yet formed and ambient GABA released from growth
cones and astrocytes is detected by high affinity subunits
(Cellot and Cherubini, 2013). Depolarizing tonic currents
facilitate spiking, thus increasing the probability of firing
coincidence from different cells and therefore of synaptic
wiring important for synaptogenesis. Moreover, these tonic
currents drive neuronal migration and maturation, axon
growth, and synaptic plasticity (Ben-Ari et al., 2007; Wang
and Kriegstein, 2009; Kilb et al., 2013; Luhmann et al., 2015).
Also GABAB Rs regulate neuronal migration, maturation of
pyramidal neurons, synapse formation and circuit development
(Fiorentino et al., 2009; Gaiarsa and Porcher, 2013). Notably,
GABAB R activation is not able to elicit GIRK-mediated
(hyperpolarizing) currents in early neurodevelopment,
although maintaining the pre-synaptic inhibitory function
on neurotransmitter release. Indeed, coupling of postsynaptic
GABAB Rs to Kir channels is delayed during development
(Fukuda et al., 1993; Gaiarsa et al., 1995; McLean et al.,
1996).
Finally, in neurodevelopment, both GABAA - and GABAB mediated transmission control synaptic plasticity. Indeed,
GABAA receptors interfere both with the opening and the
closure of the visual cortex critical period, i.e., the time
window when brain plasticity can be evoked by environmental
stimuli and experimental paradigms in slices and in vivo
(i.e., long-term potentiation (LTP) and induction and monocular
deprivation, respectively; Levelt and Hübener, 2012). Eventually,
in the adult brain, GABAA Rs usually exert a negative
regulation on hippocampal plasticity and cognition. Indeed,
GABAB Receptors
GABAB Rs are Gi/o -protein-coupled receptors (GPCRs) mostly
localized extra-synaptically. Their high affinity for GABA
ensures that ambient GABA spilled out of synapses can
activate GABAB Rs despite of their distance. In particular,
the GABAB R Gαi/o subunit can lead to adenylate cyclase
inhibition and consequent reduction of cAMP levels and PKA
pathway activity (Bettler et al., 2004; Padgett and Slesinger,
2010); Gβγi/o subunit, instead, inhibits voltage-gated calcium
channels and opens G protein-coupled inwardly-rectifying
potassium channels (GIRK/Kir3), tetramers formed by different
compositions of GIRK1–4 subunits (Koyrakh et al., 2005).
In particular, presynaptic GABAB Rs reduce vesicle release by
inhibition of the voltage-gated calcium channels and by a
calcium independent mechanism (Rost et al., 2011), whereas
GABAB R subunits found on dendrite and spine necks regulate
neuronal excitability by coupling to the GIRK channels (Nicoll,
2004; Koyrakh et al., 2005). Interestingly, all GIRK subunits
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GABAA R-mediated inhibition suppresses LTP both in vitro and
in vivo (Wigström and Gustafsson, 1986; Grover and Yan,
1999; Matsuyama et al., 2008), and benzodiazepines (positive
regulators of GABAA Rs) or GABAA R activation impairs memory
(Roth et al., 1984; Zarrindast et al., 2002; Raccuglia and Mueller,
2013). While LTP is enhanced by GABAA R blockade (Wigström
and Gustafsson, 1986), long term depression (LTD) is facilitated
by GABAA R activation in vitro (Steele and Mauk, 1999),
thus suggesting that GABAA -mediated inhibition balances the
ratio between LTP and LTD. On the other hand, GABAB Rmediated membrane hyperpolarization, inhibition of voltagegated calcium channels and of back-propagating spikes, as
well as reduction of the cAMP/PKA generally contribute
to prevent synaptic plasticity. Indeed, GABAB Rs generally
suppress LTP and memory performance, however a dual role
in plasticity regulation is mediated by auto- and heteroGABAB Rs (Davies et al., 1991; Stäubli et al., 1999). Finally,
GABAergic regulation of brain plasticity also involves adult
neurogenesis. Indeed, tonic depolarizing GABAA responses by
GABAergic Parvalbumin interneurons negatively regulate adult
neurogenesis in the dentate gyrus (DG) of the hippocampus
(Song et al., 2012; Pontes et al., 2013; Pallotto and Deprez,
2014).
Phasic GABAA -mediated transmission also ensures
simultaneous and temporally limited inhibition, able to
synchronize network activity and generate network oscillations,
according to both computational models and in vitro slice
experiments from juvenile (P14–27) and adult mice (Wang and
Buzsáki, 1996; Mann and Mody, 2010). Moreover, GABAB Rs
synchronize hippocampal network activity at low oscillation
frequency (Scanziani, 2000; Kohl and Paulsen, 2010) and
are activated during cortical up-states, contributing to their
termination (Mann et al., 2009). Finally, GABA also exerts
key roles in pathological conditions such as a number of
neurodevelopmental disorders (Ramamoorthi and Lin, 2011;
Deidda et al., 2014), epilepsy (Kaila et al., 2014a), anxiety (Nuss,
2015), and neurodegenerative diseases (e.g., AD; Li et al., 2016).
by genetic manipulation of this Mmu16 region. Specifically,
the Ts65Dn mouse (Reeves et al., 1995) is the most widely
used murine model of DS and carries an extra translocation
chromosome composed of the Mmu16 syntenic region fused
to the centromeric portion of Mmu17. This freely-segregating
extra chromosome contains 90 non-KRTAP, Hsa21 proteincoding orthologues, plus 35 protein-coding genes (deriving from
Mmu17) that are not triplicated in DS (Duchon et al., 2011;
Gupta et al., 2016). Additional DS mouse models carrying a
smaller triplication of the Mmu16 syntenic region are the Ts1Cje
and the Ts1Rhr. Ts1Cje mice are characterized by the genomic
duplication of a Mmu16 segment containing 71 Hsa21 proteincoding orthologues and translocated to the distal portion of
Mmu12 (Sago et al., 1998; Gupta et al., 2016). However, the
translocation resulted in the deletion of seven genes in the most
telomeric segment of Mmu12 (Duchon et al., 2011). Ts1Rhr
mice (Olson et al., 2004) were generated by Cre/lox chromosome
engineering and carry a tandem duplication of an even smaller
Mmu16 region comprising 29 Hsa21 protein-coding orthologues
from the so-called ‘‘DS critical region’’ (DSCR; Delabar et al.,
1993; Korenberg et al., 1994).
The vast majority of the studies on DS-related cognitive
and electrophysiological abnormalities have been performed
on the Ts65Dn mouse. Indeed, although the Ts65Dn model
still presents issues from a genetic point of view (Gardiner
et al., 2003), it recapitulates many of the phenotypic features
of the human syndrome (Dierssen, 2012; Rueda et al., 2012),
and it is currently the only mouse model used for preclinical
identification of pharmacological interventions targeting DS
cognitive impairment (Gardiner, 2014). Moreover, phenotypic
comparison of different DS mouse models has suggested
that the genes triplicated in the Ts65Dn mouse are major
responsible for DS-related cognitive abnormalities (Rueda et al.,
2012). In particular, Ts65Dn mice show severe behavioral
deficits in different learning and memory tasks, including fear
conditioning, T-maze spontaneous alternation, Morris water
maze and object recognition tests (Reeves et al., 1995; Costa
et al., 2007; Fernandez et al., 2007; Contestabile et al., 2013), and
electrophysiological alterations in both synaptic transmission
(Kleschevnikov et al., 2004, 2012b; Best et al., 2007, 2012;
Hanson et al., 2007; Mitra et al., 2012) and hippocampal
synaptic plasticity (Siarey et al., 1997, 1999; Kleschevnikov et al.,
2004; Costa and Grybko, 2005; Contestabile et al., 2013). In
addition, Ts65Dn mice display alterations in dendritic spine
morphology (Belichenko et al., 2004; Guidi et al., 2013) and
impaired neurogenesis both in the developing brain (Baxter
et al., 2000; Roper et al., 2006; Chakrabarti et al., 2007, 2010;
Contestabile et al., 2007, 2009) and in neurogenic niches of
the adult brain (Clark et al., 2006; Bianchi et al., 2010a;
Contestabile et al., 2013). Additionally, Ts65Dn mice are not
spontaneously epileptic, but show increased seizures incidence
in some experimental epilepsy paradigms (Cortez et al., 2009;
Westmark et al., 2010; Joshi et al., 2016). Finally, similarly to
DS patients, Ts65Dn mice also exhibit some sleep alterations
and hyperactivity in locomotor behavior (Escorihuela et al.,
1995; Reeves et al., 1995; Sago et al., 2000; Colas et al.,
2008).
MOUSE MODELS OF DS
Most of the current knowledge regarding alterations of the
GABAergic signaling in the DS brain has come from the study
of mouse models of DS. According to a recent study utilizing
the Vertebrate Genome Annotation (VEGA) database1 , the
human chromosome 21 (Hsa21) contains a total of 222 proteincoding genes (of which 218 map to the long arm 21q),
including two large clusters of 49 keratin-associated proteins
(KRTAPs; Gupta et al., 2016). The mouse genes orthologue
of those mapping to the long arm of Hsa21 are distributed
on three syntenic regions present on mouse chromosomes
10, 16 and 17. In particular, the distal portion of mouse
chromosome 16 (Mmu16) encompasses a large (∼28 Mb)
region that contains ∼55% of Hsa21 orthologous protein-coding
genes (Antonarakis et al., 2004; Gupta et al., 2016). Therefore,
many of the available DS mouse models have been created
1 http://vega.sanger.ac.uk
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amplified neurogenesis during embryonic development of neural
progenitor cells in the medial ganglionic eminence (MGE, where
most interneurons originate during development; Marín and
Müller, 2014), and it is more prominent for Parvalbumin and
Somatostatin-positive GABAergic interneurons (Chakrabarti
et al., 2010). In line with the increase in GABAergic
interneurons, the same authors reported also an increase in
spontaneous GABAergic postsynaptic events in CA1 pyramidal
neurons. Nevertheless, further functional analysis of GABAergic
transmission by electrophysiology did not find evidence for
alterations in the frequency of miniature inhibitory postsynaptic
currents (mIPSC, which represent activity-independent quantal
release of GABA), release probability at GABAergic synapses and
evoked GABAA transmission in the hippocampal CA1 region
of adult Ts65Dn mice (Chakrabarti et al., 2010; Best et al.,
2012). Therefore, the increase of spontaneous GABAergic events
observed in Ts65Dn mice may be due to a general enhancement
in interneuron excitability rather than a specific increase in
the number of GABAergic synapses. Indeed, further electron
microscopy studies in the temporal cortex and hippocampus of
adult Ts65Dn mice found the density of symmetric synapses
(putative GABAergic) to be unaffected (Kurt et al., 2000, 2004;
Belichenko P. V. et al., 2009). Similarly, immunohistochemical
evaluation of GABAergic terminals confirmed comparable
density between WT and Ts65Dn mice in the DG, even
if the distribution of GABAergic synapses appeared altered,
with a selective redistribution of GABAergic synapses from
the dendrite shaft and the spine heads to the spine neck of
trisomic neurons (Belichenko et al., 2004; Belichenko P. V. et al.,
2009; Kleschevnikov et al., 2012b). Such selective retribution of
GABAergic synapses to the spine neck was later also confirmed
in the DG of Ts1Cje mice (Belichenko et al., 2007). The
altered location of GABAergic synapses in DS mice may impair
synaptic input integration. Indeed, GABAergic contacts on the
spine neck may account for integration of spine-converging
inputs, in replacement of contacts on the spine head and on
the dendritic shaft that integrate inputs at local synaptic and
dendritic levels, respectively. Anyhow, the increased number
of GABAergic neurons seems not to be directly associated
with increased synaptic contacts in the DS brain. The reason
is not fully understood, but GABAergic synaptic density may
normalize in adulthood for compensatory mechanisms, after
defective brain development. Indeed, differently than in adult
Ts65Dn mice, both mIPSC frequency and evoked GABAA
transmission (but not release probability) were found larger in
the CA1 region of 2 weeks old Ts65Dn mice (Mitra et al.,
2012). Further support to the compensation hypothesis is the
observation that an increased GABAergic synaptic density was
in fact found specifically in the inner molecular layer and
granular layer of the hippocampal DG of adult Ts65Dn mice
(Martínez-Cué et al., 2013; García-Cerro et al., 2014; Mojabi
et al., 2016). Indeed, this brain area is highly innervated by
GABAergic fibers, and one could expect that developmental
compensatory mechanisms may be more difficult to put in
place. On the other hand, all these seemingly conflicting results
may simply indicate that GABAA -mediated dysfunctions are not
uniform in all areas of the trisomic brain and/or may also be
Although Ts1Cje and Ts1Rhr mice show some DS-related
phenotypes, the extent of these features is somewhat milder
and with some distinct characteristics (Rueda et al., 2012). In
particular, Ts1Cje mice displayed deficits in synaptic plasticity
and behavior in the T-maze test comparable to Ts65Dn mice,
whereas memory performances were less severely affected in the
Morris water maze and substantially spared in the novel object
recognition (Sago et al., 1998; Siarey et al., 2005; Belichenko et al.,
2007; Fernandez and Garner, 2007). On the other hand, Ts1Rhr
mice showed impaired synaptic plasticity in the hippocampal
DG, but unaffected plasticity in the CA1 hippocampal region.
At the behavior level, deficits were detected in the T-maze
and novel object recognition tests, but not in the Morris
water maze test (Olson et al., 2007; Belichenko P. N. et al.,
2009). Finally, Ts1Cje mice were found hypoactive (Sago et al.,
1998), and Ts1Rhr mice mostly not different compared to
WT animals for locomotor behavior (Belichenko P. N. et al.,
2009).
In more recent years, systematic application of Cre/loxmediated chromosome engineering has permitted the creation
of three new murine lines (Dp10, Dp16 and Dp17) individually
trisomic (through a tandem duplication on the corresponding
chromosome) for the three complete syntenic regions on
Mmu10, 16 and 17, respectively (Li et al., 2007; Yu et al.,
2010a,b). Nevertheless, although these mouse lines represent
ideal models in terms of genetic triplication, they lack the
extra freely-segregating chromosome that is found in most
individuals with DS and in Ts65Dn mice. In fact, the presence
of an extra unpaired chromosome could also have a role per se
in the phenotypic consequences of trisomy by impacting on
global gene expression and/or chromatin structure (Reinholdt
et al., 2009; Dierssen, 2012). On the other hand, mice fullytrisomic for all Hsa21 orthologue genes can also be created
by successively crossing the Dp10, Dp16 and Dp17 lines,
although decreased viability and poor breading have limited
the experimental studies on these triple trisomic mice (Yu
et al., 2010a; Belichenko et al., 2015). Moreover, the Dp10,
Dp16 and Dp17 mice have also permitted the dissection of
the relative contribution of the different triplicated regions to
the diverse disease phenotypes. Interestingly, while Dp16 and
triple trisomic mice (Dp10/Dp16/Dp17) show behavioral and
synaptic plasticity deficits comparable to the ones found in
Ts65Dn mice (Yu et al., 2010a,b; Belichenko et al., 2015), and
the single trisomic mice Dp10 as well as Dp17 show normal (or
even enhanced) performances (Yu et al., 2010b), highlighting
the importance of the Mmu16 syntenic region in DS-related
phenotype.
GABA SIGNALING IN DS MOUSE MODELS
GABAA Signaling and Trisomy
A first main finding regarding GABA-related phenotypes
in DS came from the observation that the number of
GABAergic interneurons is increased in both the cortex and
the hippocampus of Ts65Dn mice (Chakrabarti et al., 2010;
Pérez-Cremades et al., 2010; Hernández et al., 2012; HernándezGonzález et al., 2015). Specifically, this increase arises from
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simply due to age-related changes. Indeed, mIPSC frequency
and evoked GABAergic transmission were found increased in
the DG of adult Ts65Dn mice, but with this alteration primarily
attributed to increased release probability at GABAergic
terminals, rather than increased synapse number (Kleschevnikov
et al., 2004, 2012b), indicating possible sub-regional differences
in the density of GABAergic synapses and its functional
consequences (see Figure 1). Additionally, mIPSC frequency
was found decreased (rather than increased) in the hippocampal
CA3 region of Ts65Dn mice (Hanson et al., 2007; Stagni
et al., 2013). Finally, outside the hippocampus, increased
excitability, enhanced release probability and decreased tonic
FIGURE 1 | Subregion-specific GABA-related dysfunctions in the hippocampus of the Ts65Dn mouse model. IPSC, inhibitory postsynaptic currents;
sIPSC, spontaneous IPSC; mIPSC, miniature IPSC; eIPSC, evoked IPSC; freq, frequency; ampl, amplitude; mEPSC, miniature excitatory postsynaptic currents;
[Cl]I , intracellular chloride concentration; E Cl , reversal potential of GABAA R-mediated Cl currents; V REST , resting membrane potential. Arrows indicate increases or
decreases of the reported measures.
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hippocampal cultures showed decreased response to bicuculline
application on network burst-amplitude and duration in
Ts65Dn mice, as well as decreased network bursts upon GABA
application in both WT and Ts65Dn culture (Stern et al.,
2015).
Despite of whether GABAA R-induced depolarization may
reach the threshold for neuronal excitation or not, GABAA Rdriven depolarization may still work in favor of excitation
more efficiently than GABAA R-driven shunting would work in
favor of inhibition. Indeed, in Ts65Dn neurons the shunting
effect of GABAA opening may be very mild on dendritic signal
processing, since most synapses are located on the spine neck.
Instead, GABAA depolarizing PSPs may generate in the spine
neck for the depolarized ECl and propagate to spine heads
and dendritic shaft, consequently affecting NMDA receptor and
voltage-gated channel openings, and adding up to concurrent
excitatory PSPs. Notably, the concomitant hyperpolarized shift of
V REST in Ts65Dn neurons, due to GIRK2 triplication (Best et al.,
2012), may also contribute to the depolarizing action of GABA.
Anyhow, pharmacological inhibition of NKCC1 transport
activity with the specific antagonist Bumetanide completely
restored the hyperpolarizing and inhibitory action of GABA in
Ts65Dn neurons (Deidda et al., 2015b).
GABA inhibition were found in cerebellar granule cells of
Ts65Dn mice (Usowicz and Garden, 2012; Das et al., 2013;
Szemes et al., 2013).
Phasic and tonic GABA transmission is necessary to
synchronize neuronal firing and maintain physiological network
oscillation. Interestingly, gamma oscillation power is reduced in
Ts65Dn hippocampal slices, while duration and frequency of
up-states are reduced in Ts65Dn layer 4 of somatosensory cortex
slices (Hanson et al., 2013; Cramer et al., 2015).
Regarding the expression of the different GABA receptors, no
statistical difference was found in the hippocampus of Ts65Dn
mice compared to WT for α1 , α2 , α3 , α5 , γ2 subunits of GABAA R
(Belichenko P. V. et al., 2009; Best et al., 2012; Kleschevnikov
et al., 2012b). However, decreased immunostaining for β2/3
subunits of GABAA R have been reported in the DG of
3 month-old Ts65Dn mice, but not at later time points,
suggesting possible age-related changes (Belichenko P. V. et al.,
2009). Moreover, the binding of the selective α5 subtype radioligand [3 H]RO0154513 was unchanged in vivo in Ts65Dn mice
compared to WT, suggesting similar expression of GABAA Rs
containing the α5 subunit (Martínez-Cué et al., 2013).
As detailed above, GABAA R-mediated transmission
undergoes a developmental switch from depolarizing (excitatory)
to hyperpolarizing (inhibitory) during neuronal maturation and
it can shift between inhibition and excitation under some
circumstances and/or in particular cellular compartments
(Andersen et al., 1980; Staley et al., 1995; Gulledge and
Stuart, 2003; Viitanen et al., 2010; Ruusuvuori et al., 2013).
Consequently, the complex relationship of ionic movements
determined by GABAA R signaling and chloride transporters
must be highly regulated to avoid deleterious consequences on
neuronal physiology (Deidda et al., 2014). In this regard,
we have recently found that NKCC1 is upregulated in
the brain of both Ts65Dn mice and individuals with DS
(Deidda et al., 2015b). Accordingly, we found that [Cl− ]i was
increased in CA1 pyramidal neurons from Ts65Dn mice and
ECl was depolarized by 7.7 mV compared to WT neurons
(Deidda et al., 2015b). As a result, ECl was less negative than
V REST in trisomic neurons, thus predictive of outward Cl−
depolarizing currents upon GABAA R activation. Accordingly,
the mean firing frequency of individual CA1 neurons was
increased by exogenous application of GABA in trisomic
hippocampal slices, whereas it was decreased by blockade
of GABAA R-mediated transmission with bicuculline, the
opposite of what physiologically observed in WT neurons
(Deidda et al., 2015b). Therefore, despite the complex dual
excitatory and shunting/inhibitory effect of GABA following
ECl depolarization, GABAA signaling was mainly excitatory in
Ts65Dn neurons in our experimental conditions of prolonged
bath-application of the drugs in acute brain slices. In this
regard, the contribution of evoked, synaptically released or tonic
GABA signaling to the depolarizing effect of GABAA Rs has
not been studied yet in Ts65Dn slices. Similarly, experiments
are needed to evaluate in vivo the strength and direction of
GABAA transmission in DS. Moreover, little is known about
the contribution of GABAergic signaling on network dynamics
in trisomic neurons. However, one calcium imaging study on
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GABAB Signaling and Trisomy
Another direct link between DS genetic triplication and GABA
signaling came from the discovery that the KCNJ6 gene, which
encodes the subunit 2 of the GIRK channel (GIRK2/Kir3.2),
maps to Hsa21 (Ohira et al., 1997; Hattori et al., 2000). The
presence of an extra KCNJ6 copy in Ts65Dn mice leads to
overexpression of GIRK2 mRNA and protein in hippocampus,
cortex and midbrain (Harashima et al., 2006). Interestingly,
also the GIRK1 protein (which is not triplicated in DS) is
overexpressed in Ts65Dn brains, with normal levels of mRNA.
Thus, the increased GIRK1 protein expression is most likely
due to enhanced hetero-dimerization with GIRK2 subunit
(which drives GIRK subunit trafficking) and downstream
decreased protein turnover (Harashima et al., 2006). As a
consequence, GABAB /GIRK currents are increased in cultured
primary hippocampal neurons (Best et al., 2007), as well as in
CA1 pyramidal neurons and in DG granule cells from acute slices
of the Ts65Dn hippocampus (Best et al., 2012; Kleschevnikov
et al., 2012b). This increase of GABAB /GIRK signaling in
Ts65Dn neurons could strongly affect neuronal excitability
and plasticity by enhancement of GIRK-mediated shunting
inhibition, reduction of excitatory PSPs, back-propagating action
potentials, and change of neuronal passive properties (Lüscher
et al., 1997; Koyrakh et al., 2005). Anyhow, genetically restoring
GIRK2 gene dosage to disomy in Ts65Dn mice (by crossing
to GIRK2+/− mice) rescued the observed increase in GABAB triggered currents in CA1 pyramidal neurons (Joshi et al., 2016).
In agreement with triplication of KCNJ6 and the increase of
GABAB /GIRK signaling, V REST is hyperpolarized by 2.3 mV in
CA1 pyramidal neurons and by 6.2 mV in DG granule cells
(DGGC; Best et al., 2012; Kleschevnikov et al., 2012b). Of note,
whereas no statistical difference was found in the hippocampus
of Ts65Dn mice compared to WT for GABAB R subunit 1,
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The GABAergic Hypothesis in Down Syndrome
Finally, comparative evaluation of synaptic plasticity deficits
in the other DS mouse models has shown that DG-LTP
was similarly impaired and rescued by PTX application in
Ts1Cje, Ts1Rhr, and triple trisomic (Dp10/Dp16/Dp17) mice
(Belichenko et al., 2007; Belichenko P. N. et al., 2009; Belichenko
et al., 2015). Conversely, CA3-CA1 LTP was decreased in Ts1Cje,
Dp16 and triple trisomic mice, unchanged in Ts1Rhr and
Dp10 mice, and even significantly increased in Dp17 mice, but
none of these later studies evaluated the possible contribution of
GABAergic signaling (Siarey et al., 2005; Olson et al., 2007; Yu
et al., 2010a,b).
Of note, beside the two studies performed in primary
neuronal cultures (Best et al., 2007; Stern et al., 2015), all
other electrophysiological investigations of GABAergic signaling
alteration in DS mouse model have been performed on acute
brain slices. Therefore, further studies are needed to assess
whether GABAergic dysfunctions can be fully reproduced on
in vitro neuronal cultures and, most importantly, if they are also
occurring in vivo in the intact brain.
GABAB R subunit 2 was slightly decreased, most likely due to a
compensatory adaptation mechanism in response to the increase
of GIRK2 levels (Best et al., 2012; Kleschevnikov et al., 2012b).
Apart from GABAB Rs, GIRKs channels may also mediate
signaling from many other neurotransmitters including
acetylcholine, dopamine, opioid, serotonin, somatostatin
and adenosine through their metabotropic GPCRs (Lüscher
et al., 1997). Therefore, GIRK2 triplication in DS may
account for a large plethora of different effects depending
on the neurotransmitter system involved. For example,
GIRK2 channels also mediate the hypothermic response
induced by administration of the serotonin 5-HT1A and 5-HT7
receptor agonist 8-OH-DPAT (Costa et al., 2005). Indeed,
8-OH-DPAT hypothermic response is increased in Ts65Dn mice
(Stasko et al., 2006).
Synaptic Plasticity and GABA Signaling in
DS
A large body of evidence linking altered GABA physiology
to DS-related cognitive phenotypes came from the study of
hippocampal synaptic plasticity in trisomic mice. LTP and LTD
are widely accepted models of synaptic plasticity, characterized
by the strengthening or weakening (respectively) of synaptic
efficiency following specific stimulation protocols. In particular,
LTP represents the cellular correlate of memory and it is
necessary for different cognitive processes, including learning
(Lynch, 2004; Whitlock et al., 2006; Nabavi et al., 2014). In
line with the impairment in cognitive abilities characteristic of
persons with DS and of DS mouse models, synaptic plasticity
paradigms (both LTP and LTD) were found altered in Ts65Dn
hippocampal slices (Siarey et al., 1997, 1999, 2006).
Given the already described pivotal role of GABAergic
inhibition in regulating synaptic plasticity and given the defective
GABAergic transmission in DS animals, researchers have
evaluated the effect of GABAergic drugs on different forms of
hippocampal LTP in DS mice. Interestingly, LTP occurring at
Shaffer collateral-CA1 synapses (CA3-CA1 LTP) was rescued in
Ts65Dn slices by acute application in the recording bath of the
GABAA R antagonist picrotoxin (PTX). Similarly, potentiation
at perforant path-DGGC synapses (DG-LTP) was restored by
application of either PTX or the GABAB R antagonist CGP55845
(Kleschevnikov et al., 2004, 2012a; Costa and Grybko, 2005),
establishing a causal link between GABAergic transmission and
plasticity deficits in DS. Remarkably, CA3-CA1 LTP was also
rescued by application of the NKCC1 inhibitor Bumetanide,
indicating the involvement of depolarizing GABAergic signaling
in the impairment of synaptic plasticity in DS (Deidda et al.,
2015b). An even more striking evidence came about when
it was shown that DG-LTP was rescued in acute slices
from Ts65Dn mice that had been chronically treated with
the GABAA R blocker pentylenetetrazole (PTZ). The effect
was evident for up to 1–3 months after treatment cessation
(Fernandez et al., 2007). Accordingly, chronic in vivo treatment
with RO4938581, a selective negative allosteric modulator
of α5 -containing GABAA Rs, rescued in vitro CA3-CA1 LTP
in Ts65Dn mice, although it was not assessed after drug
withdrawal (Martínez-Cué et al., 2013).
Frontiers in Cellular Neuroscience | www.frontiersin.org
PHARMACOLOGICAL INTERVENTIONS
TARGETING GABA TRANSMISSION TO
RESCUE COGNITIVE DEFICITS IN DS
MOUSE MODELS
Given the large body of evidence showing the involvement of
GABA signaling in neurophysiology, cognition and synaptic
plasticity in DS, many studies have evaluated learning
and memory processes in DS mice after pharmacological
interventions targeting GABAergic transmission. Although
therapy with GABAA R antagonists may be hampered by possible
pro-epileptic and anxiogenic side effects in patients already at
increased risk for these conditions, a first seminal study evaluated
the efficacy of different GABAA R antagonists (PTX, PTZ and
Bilobalide) at non-epileptic doses on long-term declarative
memory in the novel object recognition test in Ts65Dn mice:
chronic (but not acute) blockade of GABAA transmission
rescued object recognition memory after 2-weeks of treatment
(Fernandez et al., 2007). Strikingly, the positive effect of
PTZ (a drug previously used in the clinic) was maintained
after an additional 2 months of drug withdrawal, indicating
that the treatment likely induced long-term neuronal-circuit
rearrangements able to sustain cognitive performance in trisomic
mice. The positive effect of chronic PTZ treatment on learning
and memory was further confirmed in the Morris water maze
by a later study (Rueda et al., 2008). However, the same study
showed a worsening effect of PTZ in Ts65Dn mice in a test
for equilibrium, indicating a possible side effect of the drug.
Interestingly, a follow-up study found that the effective dose of
PTZ could be reduced by 10 times without compromising its
efficacy, thus defining a potential safer therapeutic window with
respect to the possible pro-epileptic and anxiogenic side effects
of the drug (Colas et al., 2013).
Prompted by the effectiveness of PTZ treatment and by
the observation that mice knock-out for the GABAA R α5
subunit—which is highly expressed in the hippocampus (Wisden
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The GABAergic Hypothesis in Down Syndrome
Wisniewski and Kida, 1994; Raz et al., 1995; Aylward et al., 1997,
1999; Teipel et al., 2003; Teipel and Hampel, 2006; Contestabile
et al., 2007; Guidi et al., 2008). Nevertheless, only two histological
studies on DS autoptic brain samples have selectively evaluated
cell counts of bona fide GABAergic interneurons. One first
study found decreased number of Golgi-stained aspinous stellate
(putative GABAergic) cells in the somatosensory, visual and
auditory cortices (Ross et al., 1984). Accordingly, the number
of Parvalbumin and Calbindin-positive non-pyramidal neurons
was also reduced in the frontal and temporal cortices of DS
patients (Kobayashi et al., 1990). Although such decrease of
GABAergic neurons may come from a secondary effect due to
Alzheimer-like degeneration in DS, brain GABA concentration
has been shown to be specifically decreased in AD, but not
in aging DS patients (Seidl et al., 2001). Moreover, microarray
studies to evaluate changes in gene expression on human DS
cortical neuronal progenitor cells (hNPCs) in culture have shown
gene changes indicative of decreased GABAergic interneuron
genesis (Bhattacharyya et al., 2009). Moreover, the same study
showed increased expression of the α2 subunit of the GABAA R,
and downregulation of the α3 and α5 subunits in trisomic cells
compared to controls (Bhattacharyya et al., 2009). This GABAA R
composition favoring the α2 over the α5 subunits may be
indicative that GABAA R opening is fastened, possibly reducing
the opportunity of shunting inhibition and generating quick
PSPs, a condition that may be of particular relevance if GABAA mediated transmission is depolarizing in humans. Indeed, we
have shown that NKCC1 is overexpressed also in the brains
of DS patients, establishing a direct parallel with the Ts65Dn
model (Deidda et al., 2015b), and possibly suggesting depolarized
ECl in humans as in animal models. Finally, GABA levels were
found decreased or unchanged in neurochemical (Reynolds and
Warner, 1988; Seidl et al., 2001; Whittle et al., 2007) and 1 H MRS
studies (Śmigielska-Kuzia and Sobaniec, 2007; Śmigielska-Kuzia
et al., 2010) on human trisomic brains.
The recently developed technique for reprogramming somatic
cells has opened the possibility of studying patient-derived
neurons as a valuable tool for modeling neurological diseases
(Takahashi et al., 2007; Park et al., 2008). This approach has been
also applied to DS and has permitted the generation of different
induced pluripotent stem cells (iPSCs) lines. In particular,
two studies have used trisomic iPSCs to assess GABAergic
neurogenesis and synaptogenesis upon induction of neuronal
differentiation. The results from these studies have highlighted
a general impairment in synaptogenesis of DS iPSC-derived
neurons that was mirrored by a decrease in the frequency
of both inhibitory and excitatory spontaneous postsynaptic
currents (sPSCs). However, the percentage of neurons expressing
GABAergic markers, the fraction of GABAergic synapses, or the
ratio of glutamatergic to GABAergic sPSCs were substantially
unaffected (Weick et al., 2013; Hibaoui et al., 2014).
Overall, although the available data from human
studies—while limited—seem not to support the data derived
from animal research of increased GABA-mediated transmission
due to the overproduction of GABAergic interneurons, further
electrophysiological studies on iPSCs-derived neurons are
needed. On the other hand, no study has yet assessed the
et al., 1992)—show increased learning and memory performance
(Collinson et al., 2002), a second series of studies has evaluated
the efficacy of two inverse agonists selective for the α5 subunit
of the GABAA R and acting as negative allosteric modulators.
These two drugs had been developed by different pharmaceutical
companies as cognitive enhancers: α5IA (Chambers et al., 2003)
and RO4938581 (Ballard et al., 2009). Both acute and chronic
treatments with these drugs were proven effective in ameliorating
cognitive performance of Ts65Dn mice in both the novel object
recognition and Morris water maze tests without showing
pro-epileptic or anxiogenic side effects that may be associated
with GABAA R antagonism (Braudeau et al., 2011a,b; MartínezCué et al., 2013).
Moreover, following the observation that the chloride
importer NKCC1 is upregulated in DS and that GABAA
transmission is depolarizing in adult Ts65Dn mice, we
have recently assessed the efficacy of the NKCC1 inhibitor
Bumetanide (a FDA-approved loop diuretic) on learning
and memory in trisomic mice. We found that chronic
NKCC1 inhibition in Ts65Dn mice was able to rescue
discriminative memory in the object recognition test, spatial
memory in the object location test and associative memory
in the contextual fear conditioning task (Deidda et al.,
2015b). Interestingly, the effect of Bumetanide was evident
also after acute treatment and quickly lost upon treatment
cessation, indicating that the effect of Bumetanide relied
on direct NKCC1 inhibition rather than on neuronal-circuit
rearrangements. These findings are in line with previous studies
on the positive effect of GABAA R antagonists on learning
and memory in DS mice. Indeed, GABAA R antagonists will
reduce aberrant depolarizing GABAA signaling in Ts65Dn mice
regardless of whether GABAA R signaling is increased in DS.
On the other hand, since modulation of [Cl− ]i is predicted to
have only little effect on shunting inhibition, Bumetanide would
preserve the ability of GABAA currents to shunt concurrent
excitatory inputs, hence possibly reducing potential pro-epileptic
side effects. Conversely, GABAA R antagonists together with
reducing receptor transmission will also reduce the shunting
inhibition, thus possibly increasing the risk of seizures and
profoundly altering neuronal input integration.
With respect to increased GABAB R signaling in DS, both
acute and chronic treatment with the specific GABAB R
antagonist CGP55845 restored cognitive performance in the
novel object recognition test and associative memory in the
contextual fear conditioning test in Ts65Dn mice (Kleschevnikov
et al., 2012a), indicating also the possible involvement of
metabotropic GABA signaling in DS cognitive impairment.
GABA SIGNALING IN DS PATIENTS AND
PATIENT-DERIVED iPSC
Less detailed information is obviously available regarding the
GABAergic system in individuals with DS. Reduced brain size
and decreased density of neurons are hallmarks of DS and mainly
arise from reduced neurogenesis during brain development
(Colon, 1972; Sylvester, 1983; Wisniewski et al., 1984; Wisniewski
and Schmidt-Sidor, 1989; Wisniewski, 1990; Kesslak et al., 1994;
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occurrence of depolarizing GABAA signaling or increased
GABAB -mediated transmission in human DS neurons.
GABAergic DRUGS IN DS CLINICAL
TRIALS
Due to the encouraging preclinical data highlighting a strong link
between GABA signaling and DS cognitive impairment, some of
the pharmacological interventions effective in DS mouse models
have been translated into clinical trials on individuals with DS.
In particular, although the use of PTZ on DS patients has been
questioned due to the potential pro-epileptic side effects, the
dosage effective in rescuing learning and memory in Ts65Dn
mice is well below the epileptic dose (Colas et al., 2013). Indeed,
Balance Therapeutics is conducting a clinical trial in Australia to
evaluate the efficacy of PTZ (BTD-001) on individuals with DS.
This COMPOSE trial (Cognition and Memory in People with DS,
registered in the Australian-New Zealand clinical trial registry as
ACTRN12612000652875) is a phase IB study aimed at evaluating
safety, tolerability, preliminary efficacy and pharmacodynamics
of BTD-001 at two different doses in adults and adolescents with
DS. Recruitment for this study has been completed2 and the
results are eagerly awaited.
Moreover, Hoffmann-La Roche has conducted two clinical
trials for assessing the efficacy of the selective negative
allosteric modulator of the α5 -containig GABAA R Basmisanil
(RG1662/RO5186582), a derivative of RO4938581, previously
shown to rescue learning and memory in Ts65Dn mice
(Martínez-Cué et al., 2013). The first trial (CLEMATIS)
was designed as a phase II placebo-controlled study
(NCT02024789) aimed at evaluating the efficacy and safety
of RG1662 at two different doses in adults and adolescents
with DS. Disappointingly, although the complete results of
the CLEMATIS trial have not been disclosed yet, a media
release from Roche later this June has announced that the
study did not meet its primary and secondary endpoints on
improving cognitive functions in DS patients, and that there was
no significant difference between the placebo and the treated
groups3 . The lack of efficacy seen in the CLEMATIS trial induced
the discontinuation of the second ongoing placebo-controlled
dose-investigating pediatric study (NCT02484703), aimed at
evaluating pharmacokinetics, pharmacodynamics, efficacy,
and safety of RG1662 in children with DS. The interruption
was not decided for safety reasons, as the drug appeared to be
well-tolerated and no relevant side effects were observed.
SPECULATION FOR FUTURE DIRECTIONS
IN THE RESEARCH ON DS AND
GABAergic TRANSMISSION
Possible Convergence of Different Drug
Treatments on GABAergic Signaling in DS
Alternative mechanisms, apart from GABA signaling, have been
shown to underline LTP and/or cognitive deficits in Ts65Dn
2 http://www.anzctr.org.au
3 http://www.roche.com/media/store/statements.htm
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The GABAergic Hypothesis in Down Syndrome
mice. Indeed, CA3-CA1 LTP and/or behavioral performances
were rescued in Ts65Dn mice by a variety of manipulations
including: acute application of the GluN2B-selective antagonist
Ro25–6981 (Hanson et al., 2013) or of the green tea polyphenolic
compound epigallocatechin-3-gallate (EGCG, an inhibitor of
the DS triplicated kinase Dyrk1A; Xie et al., 2008), chronic
treatments with polyphenolic green tea extracts enriched in
EGCG (De la Torre et al., 2014; Catuara-Solarz et al., 2015),
the monoacylglycerol lipase inhibitor JZL184 (Lysenko et al.,
2014), the neuro-hormone Melatonin (Corrales et al., 2013), the
Sonic Hedgehog agonist SAG1.1 (Das et al., 2013), the serotonin
reuptake inhibitor Fluoxetine (Bianchi et al., 2010b; Begenisic
et al., 2014; Guidi et al., 2014), or also exposure to an enriched
environment (EE; Begenisic et al., 2011, 2015). Nevertheless,
most of these treatments are also known to directly or indirectly
modulate the GABAergic system, while for others such link has
not been established yet. For example, Ro25–6981 selectively
reduced the activation of GABAergic interneurons in the
Stratum Radiatum of the hippocampus (Hanson et al., 2013),
whereas JZL184 decreased GABAergic transmission by likely
modulating presynaptic cannabinoid receptors (Katona et al.,
1999; Zhang et al., 2009; Lee et al., 2015). Instead, both
treatment with Fluoxetine and exposure of Ts65Dn mice to
EE showed beneficial effects on LTP and memory in Ts65Dn
mice, possibly by reducing release from GABAergic terminals
(Begenisic et al., 2011, 2014, 2015). Indeed, Fluoxetine has been
previously found to reduce GABAergic neurotransmission in the
hippocampus independently from the inhibition of serotonin
reuptake (Méndez et al., 2012; Caiati and Cherubini, 2013),
and to decrease the levels of extracellular GABA in vivo
(Maya Vetencourt et al., 2008). Additionally, Fluoxetine may
also inhibit GIRK channels (Kobayashi et al., 2003; Cornelisse
et al., 2007), therefore possibly normalizing enhanced GABAB R
signaling in Ts65Dn mice. In this regard, it is important
to note that GIRK channels are also coupled to serotonin
receptors 5-HT1A (Williams et al., 1988; Llamosas et al., 2015;
Montalbano et al., 2015) and GIRK2 triplication can therefore
impact on serotoninergic signaling in DS. Indeed, stimulation
of 5-HT1A receptors in the hippocampus can reduce neuronal
firing frequency and gamma oscillations through GIRK channels
activation (Johnston et al., 2014). Since fluoxetine treatment
reduces GIRK-mediated 5-HT1A and GABAB receptor signaling
in the dorsal Raphe (Cornelisse et al., 2007), this mechanism may
be involved in the therapeutic effect of fluoxetine in Ts65Dn
mice. However, a possible link between changes in serotonin
signaling due to GIRK2 overexpression and the effects of
fluoxetine on learning and memory has not been assessed in DS
mice. On the other hand, chronic treatment with polyphenolic
green tea extracts enriched in EGCG decreased the expression
of the GABAergic synaptic markers GAD67 and VGAT in
the cortex of Ts65Dn mice (Souchet et al., 2015). Conversely,
EGCG positively modulated GABAA -mediated transmission
when administered acutely (Vignes et al., 2006; Park et al.,
2011). Of note, a recently concluded clinical trial (TESDAD,
NCT01699711) evaluating the efficacy of long-term green tea
extract treatment on DS patients showed some behavioral
improvements, although—as stated by the authors—below the
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Contestabile et al.
threshold for clinical relevance in 2 out of 15 measured tests (de
la Torre et al., 2016). Finally, the positive effect of Melatonin is
unlikely to depend on decreased GABAergic signaling because it
acts as a positive allosteric modulator of GABAA receptors (Wang
et al., 2003; Scott et al., 2010; Cheng et al., 2012). Altogether, these
data indicate that a number of the large plethora of effective drug
treatments able to restore LTP and/or memory in Ts65Dn mice
may rely on a common ground of action through the modulation
of the GABAergic system.
Excitatory Deficits in DS
Altogether, the literature reported above indicates a key role for
GABAergic signaling in neuronal network deficits in trisomic
mice. However, deficits in excitatory inputs and glutamatergic
transmission could also contribute to the imbalance in
excitatory/inhibitory transmission in the trisomic brain. Indeed,
several lines of evidence indicate delayed development and
decreased production of excitatory glutamatergic neurons in the
cortex (Chakrabarti et al., 2007, 2010; Tyler and Haydar, 2013;
Guidi et al., 2014), DG (Lorenzi and Reeves, 2006; Contestabile
et al., 2007; Bianchi et al., 2010b) and cerebellum (Baxter et al.,
2000; Roper et al., 2006; Contestabile et al., 2009) of Ts65Dn
mice. Accordingly, a decreased density of glutamatergic synapses
was found in the cortex and hippocampus of Ts65Dn mice by
electron microscopy and immunohistochemistry (Kurt et al.,
2000, 2004; Chakrabarti et al., 2007; Rueda et al., 2010; Guidi
et al., 2013; Stagni et al., 2013; García-Cerro et al., 2014), as
well as in human DS iPSCs-derived neurons (Weick et al., 2013;
Hibaoui et al., 2014). However, in vivo 1 H MRS evaluation
of different metabolites and neurotransmitters in the Ts65Dn
hippocampus showed no difference in the concentration of
either GABA or glutamate (Santin et al., 2015). Despite of the
evidence supporting a possible decrease of excitatory inputs,
few electrophysiological studies have functionally evaluated
glutamatergic signaling in trisomic mice. Decreased frequency
of miniature excitatory postsynaptic currents (mEPSC) was
found in the CA3 hippocampal region (Hanson et al., 2007;
Stagni et al., 2013), whereas a decreased ratio of postsynaptic
NMDA/AMPA-evoked responses was found in the CA1 region
of Ts65Dn mice (Das et al., 2013). Interestingly, Ts65Dn mice
show increased electrophysiological and behavioral response to
pharmacological manipulations of NMDA receptors (Costa et al.,
2007; Scott-McKean and Costa, 2011). Although a complete
mechanistic explanation behind such effect will need further
investigations (Costa, 2014), inhibition of NMDA transmission
with the noncompetitive antagonist Memantine rescued learning
and memory performance in different behavioral tests both
after acute and chronic administration in Ts65Dn mice
(Costa et al., 2007; Rueda et al., 2010; Lockrow et al.,
2011). Following these studies, two clinical trials evaluated
the efficacy of Memantine on improving cognitive functions
in DS patients. Although the drug was well-tolerated, the
results of the first study (NCT00240760) did not show any
improvement on cognitive functions (Hanney et al., 2012).
However, a pitfall of this study may be represented by
the advanced age of participants. Indeed, since DS patients
are at increased risk of developing Alzheimer degeneration
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The GABAergic Hypothesis in Down Syndrome
early in life, irreversible pathological and/or degenerative
changes may have been already in place by the time of
treatment (Costa, 2014). A second trial (NCT01112683) on
a relatively small number of young-adults with DS showed
no significant difference in the primary outcome. However,
some encouraging improvements were detected in a secondary
measure of verbal memory (Boada et al., 2012). Overall,
the lack of a comprehensive assessment of glutamatergic
functions in DS mouse models will require a more detailed
investigation to clearly evaluate the involvement of glutamatergic
signaling in excitatory/inhibitory imbalance in the trisomic
brain.
GABA in Neurodevelopment and Critical
Period Plasticity in DS
DS is widely recognized as a neurodevelopmental disorder
since many (but not all) brain deficits originate during the
embryonic and early life. Since activation of both GABAA R and
GABAB R plays a key role in brain development (Gaiarsa and
Porcher, 2013; Le Magueresse and Monyer, 2013), changes in
ambient GABA (also due to increased number of GABAergic
interneurons) may underline at least some of the brain
alterations that originate during DS fetal life and persist into
adulthood. Nevertheless, no study has so far addressed the role
of aberrant GABAergic signaling in neural circuit formation
in DS. In particular, it would be interesting to evaluate the
timing of the depolarizing/hyperpolarizing GABA switch (BenAri, 2002) and related developmental changes in GABAA Rsubunit expression (Succol et al., 2012) in trisomic mice.
On the other hand, GABAB R signaling may affect DS brain
development by modulating adenylate cyclase and calcium
channels. Triplication of GIRK2 is instead unlikely to directly
contribute to GABAB -mediated early developmental DS brain
alterations, as coupling of GABAB Rs with GIRK channels does
not occur until the second postnatal week of life in rats
(Fukuda et al., 1993; López-Bendito et al., 2003; Bony et al.,
2013). Nevertheless, the possibility of premature coupling of
GABAB Rs to GIRK2 due its overexpression in DS may still
exist.
Anyhow, future studies are also needed to investigate
whether modulating GABAergic signaling and/or intracellular
Cl− accumulation during specific developmental periods, when
brain circuits are possibly more prone to plastic changes,
may result in beneficial effects in learning and memory
that persist into adulthood. In this regard, it would be
interesting to test the long-term effects of GABAA R inhibition
(i.e., PTZ or α5 negative allosteric modulator treatments) or
of lowering [Cl− ]i with Bumetanide during brain development.
The importance of an early intervention during a likely
essential period of brain development in DS is highlighted
by the observation that different treatments (e.g., Fluoxetine,
SAG1.1 and Choline) administered during gestation or in
the early postnatal period rescued memory performances in
Ts65Dn mice later in life (Bianchi et al., 2010b; Moon
et al., 2010; Das et al., 2013; Guidi et al., 2013; Velazquez
et al., 2013; Ash et al., 2014). Nevertheless, one important
and apparently neglected aspect of early pharmacological
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Contestabile et al.
interventions is related to possible side effects of drug
treatment during development, which should in fact be carefully
considered. Indeed, similarly to the beneficial effect of the
drugs, adverse effects may persist into adulthood and may
impact on the functionality of different organs or systems. A
recent example came from the NEMO trail (NCT01434225)
regarding the use of Bumetanide on newborns with hypoxic
ischemic encephalopathy. Indeed, the trial was interrupted
because of hearing loss adverse effect (Pressler et al., 2015).
Moreover, in light of the encouraging results obtained on
Ts65Dn offsprings prenatally and perinatally treated with
Fluoxetine (Bianchi et al., 2010b; Guidi et al., 2014), a clinical
trial has been recently announced4 , aimed at assessing the
efficacy of Fluoxetine administration during the prenatal (2nd
trimester) and postnatal (up to 2 years of age) periods in
ameliorating the developmental abilities of children with DS.
However, the dosage and timing of administration should be
carefully considered in the light of the fact that Fluoxetine
administration during pregnancy has been associated to birth
defects (Reefhuis et al., 2015), and treatment in adult patients
increased seizure susceptibility (Pisani et al., 1999). Moreover,
Fluoxetine treatment in rodents during embryonic and early
postnatal life modifies the migration of cortical GABAergic
interneurons, and—later in life—increases aggression in males
as well as changes emotional and social behaviors (Kiryanova
et al., 2013; Ko et al., 2014; Frazer et al., 2015; Svirsky et al.,
2016).
Aberrant GABAergic transmission may also affect the criticalperiod plasticity in the visual cortex. Interestingly, this criticalperiod plasticity depends on the depolarizing action of GABA
during early development and its length can be extended
by reducing depolarizing GABAA signaling by treatment
with Bumetanide (Deidda et al., 2015a). Therefore, an early
intervention with Bumetanide may provide an extended window
for neuronal plasticity in DS. Although no study has assessed
critical-period plasticity in visual cortical circuits of trisomic
animals, Ts65Dn mice show deficits in cortical visual evoked
potentials (VEPs; Scott-McKean et al., 2010). Interestingly,
exposure to EE either during development or adulthood restored
cortical VEP responses in Ts65Dn mice, possibly through
modulation of GABAergic transmission (Begenisic et al., 2011,
2015).
GABA in Other DS Symptoms, Possibly
Affecting Cognition
Besides cognitive impairments, individuals with DS present a
number of other symptoms (i.e., epilepsy, sleep disorders and
anxiety) that affect the quality of their lives and may in turn
also impinge on their cognitive abilities (Pueschel et al., 1991;
Stafstrom et al., 1991; Haw et al., 1996; Carter et al., 2009; Lott
and Dierssen, 2010; Breslin et al., 2011; Rissman and Mobley,
2011; Vicari et al., 2013; Angriman et al., 2015; Edgin et al., 2015;
Robertson et al., 2015; Konstantinopoulou et al., 2016; Maris
et al., 2016). Interestingly, epilepsy, sleep disorders and anxiety
4 http://www.utsouthwestern.edu/research/fact/detail.html?studyid=STU%20
032014-006
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The GABAergic Hypothesis in Down Syndrome
have all been associated to defective GABAergic transmission
(Wagner et al., 1997; Choi et al., 2008; Rudolph and Knoflach,
2011; Möhler, 2012; Kaila et al., 2014b).
DS patients and Ts65Dn mice demonstrate increased
incidence of epileptic seizures (Stafstrom et al., 1991; Westmark
et al., 2010; Rissman and Mobley, 2011; Lott, 2012; Robertson
et al., 2015). These observations appeared contradictory when
considering that increased GABA signaling in Ts65Dn mice
was expected to overall decrease neuronal network activity,
and therefore reduce incidence of seizures. However, the
increased incidence of seizures is in line with depolarizing
GABAA signaling in DS, because the shift in GABAA R-mediated
responses will also clearly impact on the excitatory/inhibitory
balance and promote brain neuronal circuit hyperexcitability.
Interestingly, administration of γ-butyrolactone (GBL, a prodrug
for the GABAB R agonist γ-hydroxybutyrate: GHB) induced
epileptiform activity in Ts65Dn mice that was rescued by
genetically restoring GIRK2 gene dosage to disomy (Cortez
et al., 2009; Joshi et al., 2016). Although the general molecular
mechanism of GHB action is still matter of debate (Bay
et al., 2014; Venzi et al., 2015), it is intriguing to speculate
that, in the scenario of GABA dysregulation in DS, both the
GABAB R-GIRK2 signaling and depolarizing GABAA signaling
may play a role in GBL-induced epileptic phenotype seen
in Ts65Dn mice. Indeed, GHB would further increase the
already enhanced GIRK2-mediated signaling, thus abnormally
drifting V REST towards hyperpolarization. This condition would
emphasize the depolarizing GABAA signaling, with amplification
of all depolarizing inputs and consequent trigger of epileptic
activity. On the other hand, Ts65Dn mice also show increased
incidence of audiogenic seizure that can be reduced by treatment
with the metabotropic glutamate receptor subtype mGluR5
antagonist Fenobam (Westmark et al., 2010), but not by
inhibition of NKCC1 with Bumetanide (Deidda et al., 2015b).
Moreover, also the characteristic hyperactivity of Ts65Dn
mice (Escorihuela et al., 1995; Reeves et al., 1995; Sago
et al., 2000) was reduced by the α5 -containing GABAA R
negative modulator RO4938581 (Martínez-Cué et al., 2013), but
not by Bumetanide treatment (Deidda et al., 2015b). These
observations indicate that more complex mechanisms in addition
to altered GABA signaling may underline these increased
seizure susceptibility and hyperactive phenotypes in Ts65Dn
mice.
DS patients have also higher incidence of sleep disturbance
in relation to the general population (Carter et al., 2009; Breslin
et al., 2011; Edgin et al., 2015; Konstantinopoulou et al., 2016;
Maris et al., 2016), and Ts65Dn mice show some sleep alterations
mainly consisting in increased awaking and higher theta power
in sleep EEG (Colas et al., 2008). On the other hand, Ts65Dn
mice show little or no differences in circadian rhythms, which
are not altered by PTZ treatment (Stewart et al., 2007; Ruby et al.,
2010). Nevertheless, it is striking the observation that PTZ was
effective in restoring memory performances in Ts65Dn mice only
when it was administrated during the light phase of the day,
but not during the dark phase, indicating a possible different
circadian contribution of the GABAergic system on learning
and memory in DS (Colas et al., 2013). Given the recently
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Contestabile et al.
identified connection between GABA signaling and memory in
circadian arrhythmic animals (Ruby et al., 2008, 2013), more
studies are needed to uncover this possible relationship in DS.
Since GABAA signaling (and possibly Cl− homeostasis) has been
repeatedly described as a key regulator of sleep and circadian
rhythms (Wagner et al., 1997; Choi et al., 2008), it is possible that
alterations of GABA signaling and/or expression of NKCC1 may
play a role in sleep disorders in DS.
Finally, the strong involvement of GABAA signaling in
anxiety disorders (Rudolph and Knoflach, 2011; Möhler, 2012)
and the observation that DS patients show increased anxiety
(Vicari et al., 2013) may indicate that also this aspect possibly
originates, at least in part, from the alteration of GABAA
signaling. However, this issue has not been addressed yet in
DS mice. On the other hand, since GIRK2 knockout mice
show reduced anxiety-related behavior (Pravetoni and Wickman,
2008) but GABAB R knockout mice show an anxious phenotype
(Mombereau et al., 2005), coupling of GIRK channels to
metabotropic neurotransmitter receptor systems other than
GABAB Rs may play a role in regulating anxiety-related behaviors
in DS. For example the overexpression of GIRK2 in the midbrain
of Ts65Dn mice (Harashima et al., 2006) may modulate signal
transduction of dopamine receptors (Perez et al., 2006; Podda
et al., 2010; Marcott et al., 2014; Zhao et al., 2016) and
impact on different DS phenotypes including anxiety-related
symptoms (Sim et al., 2013). Finally, a possible role for altered
GABAergic transmission in impaired adult neurogenesis (Song
et al., 2012; Pallotto and Deprez, 2014) and AD (Li et al.,
2016)-which both eventually result in impaired cognition- is still
an unexplored field of research in DS (Rissman and Mobley,
2011).
CONCLUDING REMARKS
The genetic cause of DS has been unequivocally identified in
the triplication of genes located on the human chromosome
21, although the exact group of genes and the pathological
mechanisms underlying DS intellectual disability are still unclear.
Despite of the limitations in reproducing in mice the exact
genetic condition characterizing DS human pathology, the
creation of DS mouse models with construct and face validity has
been giving nevertheless a wide contribution in understanding
gene-phenotype association, DS pathological processes, and in
extending therapeutic prospects. Here, we evaluated the evidence
pointing at a role for abnormal signaling from GABAA and
GABAB receptors in the neuronal defects associated with DS,
and we considered particularly the Ts65Dn mouse model of DS,
one of the most largely used. The origin of defective GABAergic
transmission tracks back into early neurodevelopment, with
possible excitatory/inhibitory unbalance and neuronal and
plasticity impairments that persist into adulthood. However,
the weight of the GABAergic inhibitory vs. the glutamatergic
excitatory transmission in this unbalance is still unclear. Indeed,
depolarizing GABAA signaling coexists with glutamatergic
excitatory alterations in Ts65Dn mice, and excitatory/inhibitory
unbalance appears to be brain-region specific in DS mouse
models. Moreover, possible decreased GABAergic transmission
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The GABAergic Hypothesis in Down Syndrome
in human DS patients may seem to be discordant with the
increased GABAergic transmission in Ts65Dn mice, although to
date, human data are still scarce and inconclusive. Fortunately,
the recent advances in the use of human-derived iPSCs may
give a large contribution in the future understanding of DS
neuropathology, with higher translational viability. Indeed,
whether the failure of the CLEMATIS trial and limited positive
results from other clinical studies can be ascribed to inefficacy of
the drugs (despite the strong preclinical data), a low predictive
potential of the Ts65Dn mouse model may also have played a
role. For example, changes in GABAA R subunit composition
specific for human DS (Bhattacharyya et al., 2009), but lacking in
Ts65Dn mice, could account for the failure of pharmacological
treatments with a α5 selective antagonist in individuals with
DS. Since the ‘‘perfect’’ mouse model does not exist yet, it
will be worth for the future to assess the efficacy of each
new pharmacological treatment in more than one DS mouse
model, and promote parallel human and animal studies. On
the other hand, the failures of clinical trials may also rest in
the inadequacy of current neuropsychological tests in measuring
cognitive improvements in DS (Gardiner, 2010, 2014; Fernandez
and Edgin, 2016). Indeed, cognitive performance measured
by tests are mostly dependent by the integration of different
cognitive domains, thus the selective improvement of one of
them may not be detected; on the other hand, an improvement
in neuropsychological tests may not imply a perceived substantial
improvement in the daily life.
In conclusion, the data reported above clearly highlight
the multifaceted nature of DS brain abnormalities. These
alterations represent the sum of different molecular mechanisms
that most likely include impaired GABAergic transmission.
Possibly, these abnormalities originate (at least in part) during
development and lead to complex synaptic, physiological and
circuit changes, ultimately causing cognitive deficits and other
neurological manifestations. As a consequence, the still unmet
need of identifying effective pharmacological interventions
to alleviate DS-related cognitive impairment represents an
incredible complex challenge for the future. Surely, the
insights about GABA-related impairments in DS models may
be of great relevance also for other neurodevelopmental
disorders where defective GABAergic transmission (including
depolarizing GABAA signaling) may play a pathological role
(e.g., Autism, Fragile X syndrome, epilepsy, and possibly Rett
syndrome; Cellot and Cherubini, 2014; Deidda et al., 2014; He
et al., 2014; Khazipov et al., 2015).
AUTHOR CONTRIBUTIONS
AC, SM and LC contributed to the conception and writing of the
manuscript.
FUNDING
This work was supported by Jérôme Lejeune Foundation
(grants 254-CA2014A to AC, and 1266_CL2014A to LC) and
Telethon Foundation (grants GGP15043 to AC and GGP13187,
TCP15021 to LC).
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REFERENCES
Andersen, P., Dingledine, R., Gjerstad, L., Langmoen, I. A., and Laursen, A. M.
(1980). Two different responses of hippocampal pyramidal cells to application
of gamma-amino butyric acid. J. Physiol. 305, 279–296. doi: 10.1113/jphysiol.
1980.sp013363
Angriman, M., Caravale, B., Novelli, L., Ferri, R., and Bruni, O. (2015). Sleep in
children with neurodevelopmental disabilities. Neuropediatrics 46, 199–210.
doi: 10.1055/s-0035-1550151
Antonarakis, E. S., and Epstein, C. J. (2006). The challenge of Down syndrome.
Trends Mol. Med. 12, 473–479. doi: 10.1016/j.molmed.2006.08.005
Antonarakis, S. E., Lyle, R., Dermitzakis, E. T., Reymond, A., and
Deutsch, S. (2004). Chromosome 21 and Down syndrome: from
genomics to pathophysiology. Nat. Rev. Genet. 5, 725–738. doi: 10.1038/
nrg1448
Ash, J. A., Velazquez, R., Kelley, C. M., Powers, B. E., Ginsberg, S. D.,
Mufson, E. J., et al. (2014). Maternal choline supplementation improves spatial
mapping and increases basal forebrain cholinergic neuron number and size
in aged Ts65Dn mice. Neurobiol. Dis. 70, 32–42. doi: 10.1016/j.nbd.2014.
06.001
Aylward, E. H., Habbak, R., Warren, A. C., Pulsifer, M. B., Barta, P. E., Jerram, M.,
et al. (1997). Cerebellar volume in adults with Down syndrome. Arch. Neurol.
54, 209–212. doi: 10.1001/archneur.1997.00550140077016
Aylward, E. H., Li, Q., Honeycutt, N. A., Warren, A. C., Pulsifer, M. B., Barta, P. E.,
et al. (1999). MRI volumes of the hippocampus and amygdala in adults with
Down’s syndrome with and without dementia. Am. J. Psychiatry 156, 564–568.
doi: 10.1176/ajp.156.4.564
Ballard, T. M., Knoflach, F., Prinssen, E., Borroni, E., Vivian, J. A., Basile, J., et al.
(2009). RO4938581, a novel cognitive enhancer acting at GABAA α5 subunitcontaining receptors. Psychopharmacology 202, 207–223. doi: 10.1007/s00213008-1357-7
Baxter, L. L., Moran, T. H., Richtsmeier, J. T., Troncoso, J., and Reeves, R. H.
(2000). Discovery and genetic localization of Down syndrome cerebellar
phenotypes using the Ts65Dn mouse. Hum. Mol. Genet. 9, 195–202.
doi: 10.1093/hmg/9.2.195
Bay, T., Eghorn, L. F., Klein, A. B., and Wellendorph, P. (2014). GHB receptor
targets in the CNS: focus on high-affinity binding sites. Biochem. Pharmacol.
87, 220–228. doi: 10.1016/j.bcp.2013.10.028
Begenisic, T., Baroncelli, L., Sansevero, G., Milanese, M., Bonifacino, T.,
Bonanno, G., et al. (2014). Fluoxetine in adulthood normalizes GABA release
and rescues hippocampal synaptic plasticity and spatial memory in a mouse
model of Down syndrome. Neurobiol. Dis. 63, 12–19. doi: 10.1016/j.nbd.2013.
11.010
Begenisic, T., Sansevero, G., Baroncelli, L., Cioni, G., and Sale, A. (2015). Early
environmental therapy rescues brain development in a mouse model of Down
syndrome. Neurobiol. Dis. 82, 409–419. doi: 10.1016/j.nbd.2015.07.014
Begenisic, T., Spolidoro, M., Braschi, C., Baroncelli, L., Milanese, M., Pietra, G.,
et al. (2011). Environmental enrichment decreases GABAergic inhibition and
improves cognitive abilities, synaptic plasticity, and visual functions in a mouse
model of Down syndrome. Front. Cell. Neurosci. 5:29. doi: 10.3389/fncel.2011.
00029
Belichenko, N. P., Belichenko, P. V., Kleschevnikov, A. M., Salehi, A., Reeves, R. H.,
and Mobley, W. C. (2009). The ‘‘Down syndrome critical region’’ is sufficient
in the mouse model to confer behavioral, neurophysiological and synaptic
phenotypes characteristic of down syndrome. J. Neurosci. 29, 5938–5948.
doi: 10.1523/JNEUROSCI.1547-09.2009
Belichenko, P. V., Kleschevnikov, A. M., Becker, A., Wagner, G. E., Lysenko, L. V.,
Yu, E. Y., et al. (2015). Down syndrome cognitive phenotypes modeled
in mice trisomic for all HSA 21 homologues. PLoS One 10:e0134861.
doi: 10.1371/journal.pone.0134861
Belichenko, P. V., Kleschevnikov, A. M., Masliah, E., Wu, C., TakimotoKimura, R., Salehi, A., et al. (2009). Excitatory-inhibitory relationship in the
fascia dentata in the Ts65Dn mouse model of Down syndrome. J. Comp. Neurol.
512, 453–466. doi: 10.1002/cne.21895
Belichenko, P. V., Kleschevnikov, A. M., Salehi, A., Epstein, C. J., and
Mobley, W. C. (2007). Synaptic and cognitive abnormalities in mouse models
of Down syndrome: exploring genotype-phenotype relationships. J. Comp.
Neurol. 504, 329–345. doi: 10.1002/cne.21433
Frontiers in Cellular Neuroscience | www.frontiersin.org
The GABAergic Hypothesis in Down Syndrome
Belichenko, P. V., Masliah, E., Kleschevnikov, A. M., Villar, A. J., Epstein, C. J.,
Salehi, A., et al. (2004). Synaptic structural abnormalities in the Ts65Dn mouse
model of Down syndrome. J. Comp. Neurol. 480, 281–298. doi: 10.1002/cne.
20337
Ben-Ari, Y. (2002). Excitatory actions of gaba during development: the nature of
the nurture. Nat. Rev. Neurosci. 3, 728–739. doi: 10.1038/nrn920
Ben-Ari, Y., Gaiarsa, J. L., Tyzio, R., and Khazipov, R. (2007). GABA: a pioneer
transmitter that excites immature neurons and generates primitive oscillations.
Physiol. Rev. 87, 1215–1284. doi: 10.1152/physrev.00017.2006
Best, T. K., Cramer, N. P., Chakrabarti, L., Haydar, T. F., and Galdzicki, Z. (2012).
Dysfunctional hippocampal inhibition in the Ts65Dn mouse model of Down
syndrome. Exp. Neurol. 233, 749–757. doi: 10.1016/j.expneurol.2011.11.033
Best, T. K., Siarey, R. J., and Galdzicki, Z. (2007). Ts65Dn, a mouse model
of Down syndrome, exhibits increased GABAB -induced potassium current.
J. Neurophysiol. 97, 892–900. doi: 10.1152/jn.00626.2006
Bettler, B., Kaupmann, K., Mosbacher, J., and Gassmann, M. (2004). Molecular
structure and physiological functions of GABAB receptors. Physiol. Rev. 84,
835–867. doi: 10.1152/physrev.00036.2003
Bhattacharyya, A., McMillan, E., Chen, S. I., Wallace, K., and Svendsen, C. N.
(2009). A critical period in cortical interneuron neurogenesis in down
syndrome revealed by human neural progenitor cells. Dev. Neurosci. 31,
497–510. doi: 10.1159/000236899
Bianchi, P., Ciani, E., Contestabile, A., Guidi, S., and Bartesaghi, R. (2010a).
Lithium restores neurogenesis in the subventricular zone of the Ts65Dn mouse,
a model for Down syndrome. Brain Pathol. 20, 106–118. doi: 10.1111/j.17503639.2008.00246.x
Bianchi, P., Ciani, E., Guidi, S., Trazzi, S., Felice, D., Grossi, G., et al. (2010b).
Early pharmacotherapy restores neurogenesis and cognitive performance in
the Ts65Dn mouse model for Down syndrome. J. Neurosci. 30, 8769–8779.
doi: 10.1523/JNEUROSCI.0534-10.2010
Boada, R., Hutaff-Lee, C., Schrader, A., Weitzenkamp, D., Benke, T. A.,
Goldson, E. J., et al. (2012). Antagonism of NMDA receptors as a potential
treatment for Down syndrome: a pilot randomized controlled trial. Transl.
Psychiatry 2:e141. doi: 10.1038/tp.2012.66
Böhme, I., Rabe, H., and Lüddens, H. (2004). Four amino acids in the αsubunits
determine the γ-aminobutyric acid sensitivities of GABAA receptor subtypes.
J. Biol. Chem. 279, 35193–35200. doi: 10.1074/jbc.m405653200
Bony, G., Szczurkowska, J., Tamagno, I., Shelly, M., Contestabile, A., and
Cancedda, L. (2013). Non-hyperpolarizing GABAB receptor activation
regulates neuronal migration and neurite growth and specification by
cAMP/LKB1. Nat. Commun. 4:1800. doi: 10.1038/ncomms2820
Braudeau, J., Dauphinot, L., Duchon, A., Loistron, A., Dodd, R. H., Hérault, Y.,
et al. (2011a). Chronic treatment with a promnesiant GABA-A α5-selective
inverse agonist increases immediate early genes expression during memory
processing in mice and rectifies their expression levels in a Down Syndrome
mouse model. Adv. Pharmacol. Sci. 2011:153218. doi: 10.1155/2011/153218
Braudeau, J., Delatour, B., Duchon, A., Pereira, P. L., Dauphinot, L.,
De Chaumont, F., et al. (2011b). Specific targeting of the GABA-A
receptor α5 subtype by a selective inverse agonist restores cognitive
deficits in Down syndrome mice. J. Psychopharmacol. 25, 1030–1042.
doi: 10.1177/0269881111405366
Breslin, J. H., Edgin, J. O., Bootzin, R. R., Goodwin, J. L., and Nadel, L. (2011).
Parental report of sleep problems in Down syndrome. J. Intellect. Disabil. Res.
55, 1086–1091. doi: 10.1111/j.1365-2788.2011.01435.x
Caiati, M. D., and Cherubini, E. (2013). Fluoxetine impairs GABAergic signaling
in hippocampal slices from neonatal rats. Front. Cell. Neurosci. 7:63.
doi: 10.3389/fncel.2013.00063
Caraiscos, V. B., Elliott, E. M., You-Ten, K. E., Cheng, V. Y., Belelli, D.,
Newell, J. G., et al. (2004). Tonic inhibition in mouse hippocampal
CA1 pyramidal neurons is mediated by α5 subunit-containing γ-aminobutyric
acid type A receptors. Proc. Natl. Acad. Sci. U S A 101, 3662–3667.
doi: 10.1073/pnas.0307231101
Carter, M., McCaughey, E., Annaz, D., and Hill, C. M. (2009). Sleep problems in
a Down syndrome population. Arch. Dis. Child. 94, 308–310. doi: 10.1136/adc.
2008.146845
Catuara-Solarz, S., Espinosa-Carrasco, J., Erb, I., Langohr, K., Notredame, C.,
Gonzalez, J. R., et al. (2015). Principal component analysis of the effects of
environmental enrichment and (-)-epigallocatechin-3-gallate on age-associated
14
March 2017 | Volume 11 | Article 54
Contestabile et al.
learning deficits in a mouse model of Down syndrome. Front. Behav. Neurosci.
9:330. doi: 10.3389/fnbeh.2015.00330
Cellot, G., and Cherubini, E. (2013). Functional role of ambient GABA in refining
neuronal circuits early in postnatal development. Front. Neural Circuits 7:136.
doi: 10.3389/fncir.2013.00136
Cellot, G., and Cherubini, E. (2014). GABAergic signaling as therapeutic target
for autism spectrum disorders. Front. Pediatr. 2:70. doi: 10.3389/fped.2014.
00070
Chakrabarti, L., Best, T. K., Cramer, N. P., Carney, R. S. E., Isaac, J. T. R.,
Galdzicki, Z., et al. (2010). Olig1 and Olig2 triplication causes developmental
brain defects in Down syndrome. Nat. Neurosci. 13, 927–934. doi: 10.1038/
nn.2600
Chakrabarti, L., Galdzicki, Z., and Haydar, T. F. (2007). Defects in embryonic
neurogenesis and initial synapse formation in the forebrain of the
Ts65Dn mouse model of Down syndrome. J. Neurosci. 27, 11483–11495.
doi: 10.1523/JNEUROSCI.3406-07.2007
Chalifoux, J. R., and Carter, A. G. (2011). GABAB receptor modulation of voltagesensitive calcium channels in spines and dendrites. J. Neurosci. 31, 4221–4232.
doi: 10.1523/JNEUROSCI.4561-10.2011
Chambers, M. S., Atack, J. R., Broughton, H. B., Collinson, N., Cook, S.,
Dawson, G. R., et al. (2003). Identification of a novel, selective GABAA
α5 receptor inverse agonist which enhances cognition. J. Med. Chem. 46,
2227–2240. doi: 10.1021/jm020582q
Cheng, X. P., Sun, H., Ye, Z. Y., and Zhou, J. N. (2012). Melatonin modulates the
GABAergic response in cultured rat hippocampal neurons. J. Pharmacol. Sci.
119, 177–185. doi: 10.1254/jphs.11183fp
Cherubini, E., Rovira, C., Gaiarsa, J. L., Corradetti, R., and Ben-Ari, Y. (1990).
GABA mediated excitation in immature rat CA3 hippocampal neurons. Int.
J. Dev. Neurosci. 8, 481–490. doi: 10.1016/0736-5748(90)90080-l
Chiang, P.-H., Wu, P.-Y., Kuo, T.-W., Liu, Y.-C., Chan, C.-F., Chien, T.-C., et al.
(2012). GABA is depolarizing in hippocampal dentate granule cells of the
adolescent and adult rats. J. Neurosci. 32, 62–67. doi: 10.1523/JNEUROSCI.
3393-11.2012
Choi, H. J., Lee, C. J., Schroeder, A., Kim, Y. S., Jung, S. H., Kim, J. S., et al. (2008).
Excitatory actions of GABA in the suprachiasmatic nucleus. J. Neurosci. 28,
5450–5459. doi: 10.1523/JNEUROSCI.5750-07.2008
Ciruela, F., Fernández-Dueñas, V., Sahlholm, K., Fernández-Alacid, L.,
Nicolau, J. C., Watanabe, M., et al. (2010). Evidence for oligomerization
between GABAB receptors and GIRK channels containing the GIRK1 and
GIRK3 subunits. Eur. J. Neurosci. 32, 1265–1277. doi: 10.1111/j.1460-9568.
2010.07356.x
Clark, S., Schwalbe, J., Stasko, M. R., Yarowsky, P. J., and Costa, A. C. (2006).
Fluoxetine rescues deficient neurogenesis in hippocampus of the Ts65Dn
mouse model for Down syndrome. Exp. Neurol. 200, 256–261. doi: 10.1016/j.
expneurol.2006.02.005
Colas, D., Chuluun, B., Warrier, D., Blank, M., Wetmore, D. Z., Buckmaster, P.,
et al. (2013). Short-term treatment with the GABAA receptor antagonist
pentylenetetrazole produces a sustained pro-cognitive benefit in a mouse
model of Down’s syndrome. Br. J. Pharmacol. 169, 963–973. doi: 10.1111/bph.
12169
Colas, D., Valletta, J. S., Takimoto-Kimura, R., Nishino, S., Fujiki, N.,
Mobley, W. C., et al. (2008). Sleep and EEG features in genetic models of Down
syndrome. Neurobiol. Dis. 30, 1–7. doi: 10.1016/j.nbd.2007.07.014
Collinson, N., Kuenzi, F. M., Jarolimek, W., Maubach, K. A., Cothliff, R., Sur, C.,
et al. (2002). Enhanced learning and memory and altered GABAergic synaptic
transmission in mice lacking the α5 subunit of the GABAA receptor. J. Neurosci.
22, 5572–5580.
Colon, E. J. (1972). The structure of cerebral cortex in Down syndrome—A
quantitative analysis. Neuropediatrie 3, 362–376. doi: 10.1055/s-0028-1091775
Contestabile, A., Fila, T., Bartesaghi, R., and Ciani, E. (2009). Cell cycle elongation
impairs proliferation of cerebellar granule cell precursors in the Ts65Dn
Mouse, an animal model for Down syndrome. Brain Pathol. 19, 224–237.
doi: 10.1111/j.1750-3639.2008.00168.x
Contestabile, A., Fila, T., Ceccarelli, C., Bonasone, P., Bonapace, L., Santini, D.,
et al. (2007). Cell cycle alteration and decreased cell proliferation in the
hippocampal dentate gyrus and in the neocortical germinal matrix of fetuses
with Down syndrome and in Ts65Dn mice. Hippocampus 17, 665–678.
doi: 10.1002/hipo.20308
Frontiers in Cellular Neuroscience | www.frontiersin.org
The GABAergic Hypothesis in Down Syndrome
Contestabile, A., Greco, B., Ghezzi, D., Tucci, V., Benfenati, F., and Gasparini, L.
(2013). Lithium rescues synaptic plasticity and memory in Down syndrome
mice. J. Clin. Invest. 123, 348–361. doi: 10.1172/JCI64650
Cornelisse, L. N., van der Harst, J. E., Lodder, J. C., Baarendse, P. J.,
Timmerman, A. J., Mansvelder, H. D., et al. (2007). Reduced 5-HT1A- and
GABAB receptor function in dorsal raphé neurons upon chronic fluoxetine
treatment of socially stressed rats. J. Neurophysiol. 98, 196–204. doi: 10.1152/jn.
00109.2007
Corrales, A., Vidal, R., García, S., Vidal, V., Martínez, P., García, E.,
et al. (2013). Chronic melatonin treatment rescues electrophysiological and
neuromorphological deficits in a mouse model of Down syndrome. J. Pineal
Res. 56, 51–61. doi: 10.1111/jpi.12097
Cortez, M. A., Shen, L., Wu, Y., Aleem, I. S., Trepanier, C. H., Sadeghnia, H. R.,
et al. (2009). Infantile spasms and Down syndrome: a new animal model.
Pediatr. Res. 65, 499–503. doi: 10.1203/PDR.0b013e31819d9076
Costa, A. C. (2014). The glutamatergic hypothesis for Down syndrome: the
potential use of N-methyl-D-aspartate receptor antagonists to enhance
cognition and decelerate neurodegeneration. CNS Neurol. Disord. Drug Targets
13, 16–25. doi: 10.2174/18715273113126660183
Costa, A. C., and Grybko, M. J. (2005). Deficits in hippocampal CA1 LTP induced
by TBS but not HFS in the Ts65Dn mouse: a model of Down syndrome.
Neurosci. Lett. 382, 317–322. doi: 10.1016/j.neulet.2005.03.031
Costa, C. S. A., Scott-McKean, J. J., and Stasko, M. R. (2007). Acute injections
of the NMDA receptor antagonist memantine rescue performance deficits of
the Ts65Dn mouse model of Down syndrome on a fear conditioning test.
Neuropsychopharmacology 33, 1624–1632. doi: 10.1038/sj.npp.1301535
Costa, A. C., Stasko, M. R., Stoffel, M., and Scott-McKean, J. J. (2005). G-proteingated potassium (GIRK) channels containing the GIRK2 subunit are control
hubs for pharmacologically induced hypothermic responses. J. Neurosci. 25,
7801–7804. doi: 10.1523/JNEUROSCI.1699-05.2005
Coyle, J. T., Oster-Granite, M. L., and Gearhart, J. D. (1986). The neurobiologie
consequences of Down syndrome. Brain Res. Bull. 16, 773–787.
doi: 10.1016/0361-9230(86)90074-2
Cramer, N. P., Xu, X. F., Haydar, T., and Galdzicki, Z. (2015). Altered intrinsic
and network properties of neocortical neurons in the Ts65Dn mouse model of
Down syndrome. Physiol. Rep. 3:e12655. doi: 10.14814/phy2.12655
Das, I., Park, J.-M., Shin, J. H., Jeon, S. K., Lorenzi, H., Linden, D. J.,
et al. (2013). Hedgehog agonist therapy corrects structural and cognitive
deficits in a Down syndrome mouse model. Sci. Transl. Med. 5:201ra120.
doi: 10.1126/scitranslmed.3005983
David, M., Richer, M., Mamarbachi, A. M., Villeneuve, L. R., Dupré, D. J.,
and Hebert, T. E. (2006). Interactions between GABA-B1 receptors and
Kir 3 inwardly rectifying potassium channels. Cell. Signal. 18, 2172–2181.
doi: 10.1016/j.cellsig.2006.05.014
Davies, C. H., Starkey, S. J., Pozza, M. F., and Collingridge, G. L. (1991).
GABA autoreceptors regulate the induction of LTP. Nature 349, 609–611.
doi: 10.1038/349609a0
Deidda, G., Allegra, M., Cerri, C., Naskar, S., Bony, G., Zunino, G., et al. (2015a).
Early depolarizing GABA controls critical-period plasticity in the rat visual
cortex. Nat. Neurosci. 18, 87–96. doi: 10.1038/nn.3890
Deidda, G., Parrini, M., Naskar, S., Bozarth, I. F., Contestabile, A., and
Cancedda, L. (2015b). Reversing excitatory GABAA R signaling restores
synaptic plasticity and memory in a mouse model of Down syndrome. Nat.
Med. 21, 318–326. doi: 10.1038/nm.3827
Deidda, G., Bozarth, I. F., and Cancedda, L. (2014). Modulation of GABAergic
transmission in development and neurodevelopmental disorders: investigating
physiology and pathology to gain therapeutic perspectives. Front. Cell.
Neurosci. 8:119. doi: 10.3389/fncel.2014.00119
Delabar, J. M., Theophile, D., Rahmani, Z., Chettouh, Z., Blouin, J. L., Prieur, M.,
et al. (1993). Molecular mapping of twenty-four features of Down syndrome on
chromosome 21. Eur. J. Hum. Genet. 1, 114–124.
de la Torre, R., de Sola, S., Hernandez, G., Farré, M., Pujol, J., Rodriguez, J.,
et al. (2016). Safety and efficacy of cognitive training plus epigallocatechin3-gallate in young adults with Down’s syndrome (TESDAD): a double-blind,
randomised, placebo-controlled, phase 2 trial. Lancet Neurol. 15, 801–810.
doi: 10.1016/s1474-4422(16)30034-5
De la Torre, R., De Sol, A. S., Pons, M., Duchon, A., de Lagran, M. M., Farré, M.,
et al. (2014). Epigallocatechin-3-gallate, a DYRK1A inhibitor, rescues cognitive
15
March 2017 | Volume 11 | Article 54
Contestabile et al.
deficits in Down syndrome mouse models and in humans. Mol. Nutr. Food Res.
58, 278–288. doi: 10.1002/mnfr.201300325
Dierssen, M. (2012). Down syndrome: the brain in trisomic mode. Nat. Rev.
Neurosci. 13, 844–858. doi: 10.1038/nrn3314
Duchon, A., Raveau, M., Chevalier, C., Nalesso, V., Sharp, A. J., and Herault, Y.
(2011). Identification of the translocation breakpoints in the Ts65Dn and
Ts1Cje mouse lines: relevance for modeling Down syndrome. Mamm. Genome
22, 674–684. doi: 10.1007/s00335-011-9356-0
Edgin, J. O., Tooley, U., Demara, B., Nyhuis, C., Anand, P., and Spanò, G. (2015).
Sleep disturbance and expressive language development in preschool-age
children with Down syndrome. Child Dev. 86, 1984–1998. doi: 10.1111/cdev.
12443
Escorihuela, R. M., Fernández-Teruel, A., Vallina, I. F., Baamonde, C.,
Lumbreras, M. A., Dierssen, M., et al. (1995). A behavioral assessment of
Ts65Dn mice: a putative Down syndrome model. Neurosci. Lett. 199, 143–146.
doi: 10.1016/0304-3940(95)12052-6
Farrant, M., and Nusser, Z. (2005). Variations on an inhibitory theme: phasic
and tonic activation of GABAA receptors. Nat. Rev. Neurosci. 6, 215–229.
doi: 10.1038/nrn1625
Fernandez, F., and Edgin, J. O. (2016). Pharmacotherapy in Down’s syndrome:
which way forward? Lancet Neurol. 15, 776–777. doi: 10.1016/s14744422(16)30056-4
Fernandez, F., and Garner, C. C. (2007). Object recognition memory is conserved
in Ts1Cje, a mouse model of Down syndrome. Neurosci. Lett. 421, 137–141.
doi: 10.1016/j.neulet.2007.04.075
Fernandez, F., Morishita, W., Zuniga, E., Nguyen, J., Blank, M., Malenka, R. C.,
et al. (2007). Pharmacotherapy for cognitive impairment in a mouse model of
Down syndrome. Nat. Neurosci. 10, 411–413. doi: 10.1038/nn1860
Fiorentino, H., Kuczewski, N., Diabira, D., Ferrand, N., Pangalos, M. N.,
Porcher, C., et al. (2009). GABAB receptor activation triggers BDNF release and
promotes the maturation of GABAergic synapses. J. Neurosci. 29, 11650–11661.
doi: 10.1523/JNEUROSCI.3587-09.2009
Fiumelli, H., Cancedda, L., and Poo, M. M. (2005). Modulation of GABAergic
transmission by activity via postsynaptic Ca2+ -dependent regulation
of KCC2 function. Neuron 48, 773–786. doi: 10.1016/j.neuron.2005.
10.025
Fowler, C. E., Aryal, P., Suen, K. F., and Slesinger, P. A. (2007). Evidence for
association of GABAB receptors with Kir3 channels and regulators of G protein
signalling (RGS4) proteins. J. Physiol. 580, 51–65. doi: 10.1113/jphysiol.2006.
123216
Frazer, S., Otomo, K., and Dayer, A. (2015). Early-life serotonin dysregulation
affects the migration and positioning of cortical interneuron subtypes. Transl.
Psychiatry 5:e644. doi: 10.1038/tp.2015.147
Fritschy, J. M., and Panzanelli, P. (2014). GABAA receptors and plasticity of
inhibitory neurotransmission in the central nervous system. Eur. J. Neurosci.
39, 1845–1865. doi: 10.1111/ejn.12534
Fukuda, A., Mody, I., and Prince, D. A. (1993). Differential ontogenesis of
presynaptic and postsynaptic GABAB inhibition in rat somatosensory cortex.
J. Neurophysiol. 70, 448–452.
Gaiarsa, J.-L., and Porcher, C. (2013). Emerging neurotrophic role of GABAB
receptors in neuronal circuit development. Front. Cell. Neurosci. 7:206.
doi: 10.3389/fncel.2013.00206
Gaiarsa, J. L., Tseeb, V., and Ben-Ari, Y. (1995). Postnatal development of pre- and
postsynaptic GABAB-mediated inhibitions in the CA3 hippocampal region of
the rat. J. Neurophysiol. 73, 246–255.
García-Cerro, S., Martínez, P., Vidal, V., Corrales, A., Flórez, J., Vidal, R.,
et al. (2014). Overexpression of Dyrk1A is implicated in several cognitive,
electrophysiological and neuromorphological alterations found in a mouse
model of Down syndrome. PLoS One 9:e106572. doi: 10.1371/journal.pone.
0106572
Gardiner, K. J. (2010). Molecular basis of pharmacotherapies for cognition in
Down syndrome. Trends Pharmacol. Sci. 31, 66–73. doi: 10.1016/j.tips.2009.
10.010
Gardiner, K. J. (2014). Pharmacological approaches to improving cognitive
function in Down syndrome: current status and considerations. Drug Des.
Devel. Ther. 9, 103–125. doi: 10.2147/DDDT.S51476
Gardiner, K., Fortna, A., Bechtel, L., and Davisson, M. T. (2003). Mouse models of
Down syndrome: how useful can they be? Comparison of the gene content of
Frontiers in Cellular Neuroscience | www.frontiersin.org
The GABAergic Hypothesis in Down Syndrome
human chromosome 21 with orthologous mouse genomic regions. Gene 318,
137–147. doi: 10.1016/s0378-1119(03)00769-8
Gassmann, M., and Bettler, B. (2012). Regulation of neuronal GABAB
receptor functions by subunit composition. Nat. Rev. Neurosci. 13, 380–394.
doi: 10.1038/nrn3249
Grover, L. M., and Yan, C. (1999). Blockade of GABAA receptors facilitates
induction of NMDA receptor-independent long-term potentiation.
J. Neurophysiol. 81, 2814–2822.
Guidi, S., Bonasoni, P., Caccarelli, C., Santini, D., Gualtieri, F., Ciani, E., et al.
(2008). Neurogenesis impairment and increased cell death reduce total neuron
number in the hippocampal region of fetuses with Down syndrome. Brain
Pathol. 18, 180–197. doi: 10.1111/j.1750-3639.2007.00113.x
Guidi, S., Stagni, F., Bianchi, P., Ciani, E., Giacomini, A., De Franceschi, M.,
et al. (2014). Prenatal pharmacotherapy rescues brain development in a Down’s
syndrome mouse model. Brain 137, 380–401. doi: 10.1093/brain/awt340
Guidi, S., Stagni, F., Bianchi, P., Ciani, E., Ragazzi, E., Trazzi, S., et al. (2013). Early
pharmacotherapy with fluoxetine rescues dendritic pathology in the Ts65Dn
mouse model of Down syndrome. Brain Pathol. 23, 129–143. doi: 10.1111/j.
1750-3639.2012.00624.x
Gulledge, A. T., and Stuart, G. J. (2003). Excitatory actions of GABA in the cortex.
Neuron 37, 299–309. doi: 10.1016/s0896-6273(02)01146-7
Gupta, M., Dhanasekaran, A. R., and Gardiner, K. J. (2016). Mouse models
of Down syndrome: gene content and consequences. Mamm. Genome 27,
538–555. doi: 10.1007/s00335-016-9661-8
Hanney, M., Prasher, V., Williams, N., Jones, E. L., Aarsland, D., Corbett, A., et al.
(2012). Memantine for dementia in adults older than 40 years with Down’s
syndrome (MEADOWS): a randomised, double-blind, placebo-controlled trial.
Lancet 379, 528–536. doi: 10.1016/S0140-6736(11)61676-0
Hanson, J. E., Blank, M., Valenzuela, R. A., Garner, C. C., and Madison, D. V.
(2007). The functional nature of synaptic circuitry is altered in area CA3 of the
hippocampus in a mouse model of Down’s syndrome. J. Physiol. 579, 53–67.
doi: 10.1113/jphysiol.2006.114868
Hanson, J. E., Weber, M., Meilandt, W. J., Wu, T., Luu, T., Deng, L., et al.
(2013). GluN2B antagonism affects interneurons and leads to immediate
and persistent changes in synaptic plasticity, oscillations, and behavior.
Neuropsychopharmacology 38, 1221–1233. doi: 10.1038/npp.2013.19
Harashima, C., Jacobowitz, D. M., Witta, J., Borke, R. C., Best, T. K., Siarey, R. J.,
et al. (2006). Abnormal expression of the G-protein-activated inwardly
rectifying potassium channel 2 (GIRK2) in hippocampus, frontal cortex and
substantia nigra of Ts65Dn mouse: a model of Down syndrome. J. Comp.
Neurol. 494, 815–833. doi: 10.1002/cne.20844
Hattori, M., Fujiyama, A., Taylor, T. D., Watanabe, H., Yada, T., Park, H. S., et al.
(2000). The DNA sequence of human chromosome 21. Nature 405, 311–319.
doi: 10.1038/35012518
Haw, C. M., Barnes, T. R., Clark, K., Crichton, P., and Kohen, D. (1996).
Movement disorder in Down’s syndrome: a possible marker of the severity of
mental handicap. Mov. Disord. 11, 395–403. doi: 10.1002/mds.870110408
He, Q., Nomura, T., Xu, J., and Contractor, A. (2014). The developmental switch
in GABA polarity is delayed in fragile X mice. J. Neurosci. 34, 446–450.
doi: 10.1523/JNEUROSCI.4447-13.2014
Hernández, S., Gilabert-Juan, J., Blasco-Ibáñez, J. M., Crespo, C., Nácher, J.,
and Varea, E. (2012). Altered expression of neuropeptides in the primary
somatosensory cortex of the Down syndrome model Ts65Dn. Neuropeptides
46, 29–37. doi: 10.1016/j.npep.2011.10.002
Hernández-González, S., Ballestin, R., López-Hidalgo, R., Gilabert-Juan, J., BlascoIbáñez, J. M., Crespo, C., et al. (2015). Altered distribution of hippocampal
interneurons in the murine Down syndrome model Ts65Dn. Neurochem. Res.
40, 151–164. doi: 10.1007/s11064-014-1479-8
Hibaoui, Y., Grad, I., Letourneau, A., Sailani, M. R., Dahoun, S., Santoni, F. A., et al.
(2014). Modelling and rescuing neurodevelopmental defect of Down syndrome
using induced pluripotent stem cells from monozygotic twins discordant for
trisomy 21. EMBO Mol. Med. 6, 259–277. doi: 10.1002/emmm.201302848
Johnston, A., McBain, C. J., and Fisahn, A. (2014). 5-Hydroxytryptamine1A
receptor-activation hyperpolarizes pyramidal cells and suppresses
hippocampal gamma oscillations via Kir3 channel activation. J. Physiol.
592, 4187–4199. doi: 10.1113/jphysiol.2014.279083
Jonas, P., and Buzsaki, G. (2007). Neural inhibition. Scholarpedia 2:3286.
doi: 10.4249/scholarpedia.3286
16
March 2017 | Volume 11 | Article 54
Contestabile et al.
Joshi, K., Shen, L., Michaeli, A., Salter, M., Thibault-Messier, G., Hashmi, S.,
et al. (2016). Infantile spasms in down syndrome: rescue by knockdown of the
GIRK2 channel. Ann. Neurol. 80, 511–521. doi: 10.1002/ana.24749
Kaila, K., Price, T. J., Payne, J. A., Puskarjov, M., and Voipio, J. (2014a). Cationchloride cotransporters in neuronal development, plasticity and disease. Nat.
Rev. Neurosci. 15, 637–654. doi: 10.1038/nrn3819
Kaila, K., Ruusuvuori, E., Seja, P., Voipio, J., and Puskarjov, M. (2014b). GABA
actions and ionic plasticity in epilepsy. Curr. Opin. Neurobiol. 26, 34–41.
doi: 10.1016/j.conb.2013.11.004
Katona, I., Sperlágh, B., Sík, A., Käfalvi, A., Vizi, E. S., Mackie, K., et al. (1999).
Presynaptically located CB1 cannabinoid receptors regulate GABA release
from axon terminals of specific hippocampal interneurons. J. Neurosci. 19,
4544–4558.
Kesslak, J. P., Nagata, S. F., Lott, I., and Nalciouglu, O. (1994). Magnetic resonance
imaging analysis of age-related changes in the brains of individuals with
Down’s syndrome. Neurology 44, 1039–1045. doi: 10.1212/wnl.44.6.1039
Khazipov, R., Valeeva, G., and Khalilov, I. (2015). Depolarizing GABA and
developmental epilepsies. CNS Neurosci. Ther. 21, 83–91. doi: 10.1111/cns.
12353
Khirug, S., Yamada, J., Afzalov, R., Voipio, J., Khiroug, L., and Kaila, K. (2008).
GABAergic depolarization of the axon initial segment in cortical principal
neurons is caused by the Na-K-2Cl cotransporter NKCC1. J. Neurosci. 28,
4635–4639. doi: 10.1523/JNEUROSCI.0908-08.2008
Kilb, W., Kirischuk, S., and Luhmann, H. J. (2013). Role of tonic GABAergic
currents during pre- and early postnatal rodent development. Front. Neural
Circuits 7:139. doi: 10.3389/fncir.2013.00139
Kiryanova, V., McAllister, B. B., and Dyck, R. H. (2013). Long-term outcomes of
developmental exposure to fluoxetine: a review of the animal literature. Dev.
Neurosci. 35, 437–439. doi: 10.1159/000355709
Kleschevnikov, A. M., Belichenko, P. V., Faizi, M., Jacobs, L. F., Htun, K.,
Shamloo, M., et al. (2012a). Deficits in cognition and synaptic plasticity in a
mouse model of Down syndrome ameliorated by GABAB receptor antagonists.
J. Neurosci. 32, 9217–9227. doi: 10.1523/JNEUROSCI.1673-12.2012
Kleschevnikov, A. M., Belichenko, P. V., Gall, J., George, L., Nosheny, R.,
Maloney, M. T., et al. (2012b). Increased efficiency of the GABAA and GABAB
receptor-mediated neurotransmission in the Ts65Dn mouse model of Down
syndrome. Neurobiol. Dis. 45, 683–691. doi: 10.1016/j.nbd.2011.10.009
Kleschevnikov, A. M., Belichenko, P. V., Villar, A. J., Epstein, C. J., Malenka, R. C.,
and Mobley, W. C. (2004). Hippocampal long-term potentiation suppressed by
increased inhibition in the Ts65Dn mouse, a genetic model of Down syndrome.
J. Neurosci. 24, 8153–8160. doi: 10.1523/jneurosci.1766-04.2004
Ko, M. C., Lee, L. J., Li, Y., and Lee, L. J. (2014). Long-term consequences of
neonatal fluoxetine exposure in adult rats. Dev. Neurobiol. 74, 1038–1051.
doi: 10.1002/dneu.22185
Kobayashi, K., Emson, P. C., Mountjoy, C. Q., Thornton, S. N., Lawson, D. E.,
and Mann, D. M. (1990). Cerebral cortical calbindin D28K and parvalbumin
neurones in Down’s syndrome. Neurosci. Lett. 113, 17–22. doi: 10.1016/03043940(90)90487-t
Kobayashi, T., Washiyama, K., and Ikeda, K. (2003). Inhibition of G proteinactivated inwardly rectifying K+ channels by fluoxetine (Prozac). Br.
J. Pharmacol. 138, 1119–1128. doi: 10.1038/sj.bjp.0705172
Kohl, M. M., and Paulsen, O. (2010). The roles of GABAB receptors in
cortical network activity. Adv. Pharmacol. 58, 205–229. doi: 10.1016/S10543589(10)58009-8
Konstantinopoulou, S., Tapia, I. E., Kim, J. Y., Xanthopoulos, M. S., Radcliffe, J.,
Cohen, M. S., et al. (2016). Relationship between obstructive sleep apnea
cardiac complications and sleepiness in children with Down syndrome. Sleep
Med. 17, 18–24. doi: 10.1016/j.sleep.2015.09.014
Korenberg, J. R., Chen, X. N., Schipper, R., Sun, Z., Gonsky, R., Gerwehr, S.,
et al. (1994). Down syndrome phenotypes: the consequences of chromosomal
imbalance. Proc. Natl. Acad. Sci. U S A 91, 4997–5001. doi: 10.1073/pnas.91.
11.4997
Koyrakh, L., Luján, R., Colón, J., Karschin, C., Kurachi, Y., Karschin, A., et al.
(2005). Molecular and cellular diversity of neuronal G-protein-gated potassium
channels. J. Neurosci. 25, 11468–11478. doi: 10.1523/JNEUROSCI.348405.2005
Kurt, M. A., Davies, D. C., Kidd, M., Dierssen, M., and Florez, J.
(2000). Synaptic deficit in the temporal cortex of partial trisomy 16
Frontiers in Cellular Neuroscience | www.frontiersin.org
The GABAergic Hypothesis in Down Syndrome
(Ts65Dn) mice. Brain Res. 858, 191–197. doi: 10.1016/s0006-8993(00)
01984-3
Kurt, A. M., Kafa, I. M., Dierssen, M., and Davies, C. D. (2004). Deficits of neuronal
density in CA1 and synaptic density in the dentate gyrus, CA3 and CA1, in
a mouse model of Down syndrome. Brain Res. 1022, 101–109. doi: 10.1016/j.
brainres.2004.06.075
Lamsa, K., and Taira, T. (2003). Use-dependent shift from inhibitory to excitatory
GABAA receptor action in SP-O interneurons in the rat hippocampal
CA3 area. J. Neurophysiol. 90, 1983–1995. doi: 10.1152/jn.00060.2003
Larsen, K. B., Laursen, H., Græmb, N., Samuelsena, G. B., Bogdanovicc, N., and
Pakkenberga, B. (2008). Reduced cell number in the neocortical part of the
human fetal brain in Down syndrome. Ann. Anat. 190, 421–427. doi: 10.1016/j.
aanat.2008.05.007
Lee, S. H., Ledri, M., Tóth, B., Marchionni, I., Henstridge, C. M., Dudok, B.,
et al. (2015). Multiple forms of Endocannabinoid and endovanilloid signaling
regulate the tonic control of GABA release. J. Neurosci. 35, 10039–10057.
doi: 10.1523/JNEUROSCI.4112-14.2015
Leinekugel, X., Medina, I., Khalilov, I., Ben-Ari, Y., and Khazipov, R. (1997).
Ca2+ oscillations mediated by the synergistic excitatory actions of GABAA
and NMDA receptors in the neonatal hippocampus. Neuron 18, 243–255.
doi: 10.1016/s0896-6273(00)80265-2
Leinekugel, X., Tseeb, V., Ben-Ari, Y., and Bregestovski, P. (1995). Synaptic
GABAA activation induces Ca2+ rise in pyramidal cells and interneurons from
rat neonatal hippocampal slices. J. Physiol. 487, 319–329. doi: 10.1113/jphysiol.
1995.sp020882
Le Magueresse, C., and Monyer, H. (2013). GABAergic interneurons shape the
functional maturation of the cortex. Neuron 77, 388–405. doi: 10.1016/j.neuron.
2013.01.011
Leung, L. S., and Peloquin, P. (2006). GABAB receptors inhibit backpropagating
dendritic spikes in hippocampal CA1 pyramidal cells in vivo. Hippocampus 16,
388–407. doi: 10.1002/hipo.20168
Levelt, C. N., and Hübener, M. (2012). Critical-period plasticity in the visual
cortex. Annu. Rev. Neurosci. 35, 309–330. doi: 10.1146/annurev-neuro-061010113813
Leverenz, J. B., and Raskind, M. A. (1998). Early amyloid deposition in the medial
temporal lobe of young Down syndrome patients: a regional quantitative
analysis. Exp. Neurol. 150, 296–304. doi: 10.1006/exnr.1997.6777
Li, Y., Sun, H., Chen, Z., Xu, H., Bu, G., and Zheng, H. (2016). Implications of
GABAergic neurotransmission in Alzheimer’s disease. Front. Aging Neurosci.
8:31. doi: 10.3389/fnagi.2016.00031
Li, Z., Yu, T., Morishima, M., Pao, A., Laduca, J., Conroy, J., et al.
(2007). Duplication of the entire 22.9 Mb human chromosome 21 syntenic
region on mouse chromosome 16 causes cardiovascular and gastrointestinal
abnormalities. Hum. Mol. Genet. 16, 1359–1366. doi: 10.1093/hmg/ddm086
Llamosas, N., Bruzos-Cidón, C., Rodríguez, J. J., Ugedo, L., and Torrecilla, M.
(2015). Deletion of GIRK2 Subunit of GIRK channels alters the 5-HT1A
receptor-mediated signaling and results in a depression-resistant behavior. Int.
J. Neuropsychopharmacol. 18:pyv051. doi: 10.1093/ijnp/pyv051
Lockrow, J., Bogera, H., Bimonte-Nelson, H., and Granholm, A. C. (2011). Effects
of long-term memantine on memory and neuropathology in Ts65Dn mice, a
model for Down syndrome. Behav. Brain Res. 221, 610–622. doi: 10.1016/j.bbr.
2010.03.036
López-Bendito, G., Luján, R., Shigemoto, R., Ganter, P., Paulsen, O., and
Molnár, Z. (2003). Blockade of GABAB receptors alters the tangential
migration of cortical neurons. Cereb. Cortex 13, 932–942. doi: 10.
1093/cercor/13.9.932
Lorenzi, H. A., and Reeves, R. H. (2006). Hippocampal hypocellularity in the
Ts65Dn mouse originates early in development. Brain Res. 1104, 153–159.
doi: 10.1016/j.brainres.2006.05.022
Lott, I. T. (2012). Neurological phenotypes for Down syndrome across the
life span. Prog. Brain Res. 197, 101–121. doi: 10.1016/b978-0-444-54299-1.
00006-6
Lott, I., and Dierssen, M. (2010). Cognitive deficits and associated neurological
complications in individuals with Down’s syndrome. Lancet Neurol. 9,
623–633. doi: 10.1016/s1474-4422(10)70112-5
Luhmann, H. J., Fukuda, A., and Kilb, W. (2015). Control of cortical
neuronal migration by glutamate and GABA. Front. Cell. Neurosci. 9:4.
doi: 10.3389/fncel.2015.00004
17
March 2017 | Volume 11 | Article 54
Contestabile et al.
Lüscher, C., Jan, L. Y., Stoffel, M., Malenka, R. C., and Nicoll, R. A.
(1997). G protein-coupled inwardly rectifying K+ channels (GIRKs)
mediate postsynaptic but not presynaptic transmitter actions in
hippocampal neurons. Neuron 19, 687–695. doi: 10.1016/s0896-6273(00)
80381-5
Lynch, M. A. (2004). Long-term potentiation and memory. Physiol. Rev. 84,
87–136. doi: 10.1152/physrev.00014.2003
Lysenko, L. V., Kim, J., Henry, C., Tyrtyshnaia, A., Kohnz, R. A., Madamba, F.,
et al. (2014). Monoacylglycerol lipase inhibitor JZL184 improves behavior and
neural properties in Ts65Dn mice, a model of down syndrome. PLoS One
9:e114521. doi: 10.1371/journal.pone.0114521
Mann, D. M., and Esiri, M. M. (1989). The pattern of acquisition of
plaques and tangles in the brains of patients under 50 years of age with
Down’s syndrome. J. Neurol. Sci. 89, 169–179. doi: 10.1016/0022-510x(89)
90019-1
Mann, E. O., Kohl, M. M., and Paulsen, O. (2009). Distinct roles of GABAA
and GABAB receptors in balancing and terminating persistent cortical activity.
J. Neurosci. 29, 7513–7518. doi: 10.1523/JNEUROSCI.6162-08.2009
Mann, E. O., and Mody, I. (2010). Control of hippocampal gamma oscillation
frequency by tonic inhibition and excitation of interneurons. Nat. Neurosci. 13,
205–212. doi: 10.1038/nn.2464
Marcott, P. F., Mamaligas, A. A., and Ford, C. P. (2014). Phasic dopamine
release drives rapid activation of striatal D2-receptors. Neuron 84, 164–176.
doi: 10.1016/j.neuron.2014.08.058
Marín, O., and Müller, U. (2014). Lineage origins of GABAergic versus
glutamatergic neurons in the neocortex. Curr. Opin. Neurobiol. 26, 132–141.
doi: 10.1016/j.conb.2014.01.015
Maris, M., Verhulst, S., Wojciechowski, M., Van De Heyning, P., and
Boudewyns, A. (2016). Prevalence of obstructive sleep apnea in children with
down syndrome. Sleep 39, 699–704. doi: 10.5665/sleep.5554
Martínez-Cué, C., Martínez, P., Rueda, N., Vidal, R., García, S., Vidal, V., et al.
(2013). Reducing GABAA α5 receptor-mediated inhibition rescues functional
and neuromorphological deficits in a mouse model of down syndrome.
J. Neurosci. 33, 3953–3966. doi: 10.1523/JNEUROSCI.1203-12.2013
Matsuyama, S., Taniguchi, T., Kadoyama, K., and Matsumoto, A. (2008). Longterm potentiation-like facilitation through GABAA receptor blockade in the
mouse dentate gyrus in vivo. Neuroreport 19, 1809–1813. doi: 10.1097/WNR.
0b013e328319ab94
Maya Vetencourt, J. F., Sale, A., Viegi, A., Baroncelli, L., De Pasquale, R.,
O’Leary, O. F., et al. (2008). The antidepressant fluoxetine restores plasticity
in the adult visual cortex. Science 320, 385–388. doi: 10.1126/science.
1150516
McLean, H. A., Caillard, O., Khazipov, R., Ben-Ari, Y., and Gaiarsa, J. L.
(1996). Spontaneous release of GABA activates GABAB receptors and controls
network activity in the neonatal rat hippocampus. J. Neurophysiol. 76,
1036–1046.
Megías, M., Emri, Z., Freund, T. F., and Gulyás, A. I. (2001). Total number
and distribution of inhibitory and excitatory synapses on hippocampal
CA1 pyramidal cells. Neuroscience 102, 527–540. doi: 10.1016/s03064522(00)00496-6
Méndez, P., Pazienti, A., Szabó, G., and Bacci, A. (2012). Direct alteration of a
specific inhibitory circuit of the hippocampus by antidepressants. J. Neurosci.
32, 16616–16628. doi: 10.1523/JNEUROSCI.1720-12.2012
Mitra, A., Blank, M., and Madison, D. V. (2012). Developmentally altered
inhibition in Ts65Dn, a mouse model of Down syndrome. Brain Res. 1440, 1–8.
doi: 10.1016/j.brainres.2011.12.034
Möhler, H. (2012). The GABA system in anxiety and depression and its therapeutic
potential. Neuropharmacology 62, 42–53. doi: 10.1016/j.neuropharm.2011.
08.040
Mojabi, F. S., Fahimi, A., Moghadam, S., Moghadam, S., Windy
McNerneny, M. W., Ponnusamy, R., et al. (2016). GABAergic hyperinnervation
of dentate granule cells in the Ts65Dn mouse model of down syndrome
exploring the role of app. Hippocampus 26, 1641–1654. doi: 10.1002/hipo.
22664
Mombereau, C., Kaupmann, K., Gassmann, M., Bettler, B., van der Putten, H.,
and Cryan, J. F. (2005). Altered anxiety and depression-related behaviour
in mice lacking GABAB2 receptor subunits. Neuroreport 16, 307–310.
doi: 10.1097/00001756-200502280-00021
Frontiers in Cellular Neuroscience | www.frontiersin.org
The GABAergic Hypothesis in Down Syndrome
Montalbano, A., Corradetti, R., and Mlinar, B. (2015). Pharmacological
characterization of 5-HT1A autoreceptor-coupled GIRK channels in rat dorsal
raphe 5-HT neurons. PLoS One 10:e0140369. doi: 10.1371/journal.pone.
0140369
Moon, J., Chen, M., Gandhy, S. U., Strawderman, M., Levitsky, D. A.,
Maclean, K. N., et al. (2010). Perinatal choline supplementation improves
cognitive functioning and emotion regulation in the Ts65Dn mouse model
of Down syndrome. Behav. Neurosci. 124, 346–361. doi: 10.1037/a00
19590
Nabavi, S., Fox, R., Proulx, C. D., Lin, J. Y., Tsien, R. Y., and Malinow, R.
(2014). Engineering a memory with LTD and LTP. Nature 511, 348–352.
doi: 10.1038/nature13294
Nadel, L. (2003). Down’s syndrome: a genetic disorder in biobehavioral
perspective. Genes Brain Behav. 2, 156–166. doi: 10.1034/j.1601-183x.2003.
00026.x
Nicoll, R. A. (2004). My close encounter with GABAB receptors. Biochem.
Pharmacol. 68, 1667–1674. doi: 10.1016/j.bcp.2004.07.024
Nuss, P. (2015). Anxiety disorders and GABA neurotransmission: a disturbance of
modulation. Neuropsychiatr. Dis. Treat. 11, 165–175. doi: 10.2147/NDT.s58841
Ohira, M., Seki, N., Nagase, T., Suzuki, E., Nomura, N., Ohara, O., et al.
(1997). Gene identification in 1.6-Mb region of the Down syndrome
region on chromosome 21. Genome Res. 7, 47–58. doi: 10.1101/gr.
7.1.47
Olsen, R. W., and Sieghart, W. (2008). International union of pharmacology.
LXX. Subtypes of γ-aminobutyric acidA receptors: classification on the basis
of subunit composition, pharmacology and function. Update. Pharmacol. Rev.
60, 243–260. doi: 10.1124/pr.108.00505
Olson, L. E., Richtsmeier, J. T., Leszl, J., and Reeves, R. H. (2004). A chromosome
21 critical region does not cause specific down syndrome phenotypes. Science
306, 687–690. doi: 10.1126/science.1098992
Olson, L. E., Roper, R. J., Sengstaken, C. L., Peterson, E. A., Aquino, V.,
Galdzicki, Z., et al. (2007). Trisomy for the Down syndrome ‘critical region’ is
necessary but not sufficient for brain phenotypes of trisomic mice. Hum. Mol.
Genet. 16, 774–782. doi: 10.1093/hmg/ddm022
Padgett, C. L., and Slesinger, P. A. (2010). GABAB receptor coupling to G-proteins
and ion channels. Adv. Pharmacol. 58, 123–147. doi: 10.1016/S10543589(10)58006-2
Pallotto, M., and Deprez, F. (2014). Regulation of adult neurogenesis by
GABAergic transmission: signaling beyond GABAA -receptors. Front. Cell.
Neurosci. 8:166. doi: 10.3389/fncel.2014.00166
Park, I., Arora, N., Huo, H., Maherali, N., Ahfeldt, T., Shimamura, A., et al.
(2008). Disease-specific induced pluripotent stem cells. Cell 134, 877–886.
doi: 10.1016/j.cell.2008.07.041
Park, K. S., Han, J. Y., Moon, D. C., Hong, J. T., and Oh, K. W. (2011).
(-)-Epigallocatechin-3-O-gallate augments pentobarbital-induced sleeping
behaviors through Cl− channel activation. J. Med. Food 14, 1456–1462.
doi: 10.1089/jmf.2010.1529
Pennington, B. F., Moon, J., Edgin, J., Stedron, J., and Nadel, L. (2003). The
neuropsychology of Down syndrome: evidence for hippocampal dysfunction.
Child Dev. 74, 75–93. doi: 10.1111/1467-8624.00522
Perez, M. F., White, F. J., and Hu, X. T. (2006). Dopamine D2 receptor modulation
of K+ channel activity regulates excitability of nucleus accumbens neurons at
different membrane potentials. J. Neurophysiol. 96, 2217–2228. doi: 10.1152/jn.
00254.2006
Pérez-Cremades, D., Hernández, S., Blasco-Ibáñez, J. M., Crespo, C., Nacher, J.,
and Varea, E. (2010). Alteration of inhibitory circuits in the somatosensory
cortex of Ts65Dn mice, a model for Down’s syndrome. J. Neural Transm.
(Vienna) 117, 445–455. doi: 10.1007/s00702-010-0376-9
Pinter, J. D., Eliez, S., Schmitt, J. E., Capone, G. T., and Reiss, A. L. (2001).
Neuroanatomy of Down’s syndrome: a high-resolution MRI study. Am.
J. Psychiatry 158, 1659–1665. doi: 10.1176/appi.ajp.158.10.1659
Pisani, F., Spina, E., and Oteri, G. (1999). Antidepressant drugs and seizure
susceptibility: from in vitro data to clinical practice. Epilepsia 10, S48–S56.
doi: 10.1111/j.1528-1157.1999.tb00885.x
Podda, M. V., Riccardi, E., D’Ascenzo, M., Azzena, G. B., and Grassi, C.
(2010). Dopamine D1-like receptor activation depolarizes medium spiny
neurons of the mouse nucleus accumbens by inhibiting inwardly rectifying
K+ currents through a cAMP-dependent protein kinase A-independent
18
March 2017 | Volume 11 | Article 54
Contestabile et al.
mechanism. Neuroscience 167, 678–690. doi: 10.1016/j.neuroscience.2010.02.
075
Pontes, A., Zhang, Y., and Hu, W. (2013). Novel functions of GABA signaling in
adult neurogenesis. Front. Biol. 8, 496–507. doi: 10.1007/s11515-013-1270-2
Pouille, F., and Scanziani, M. (2001). Enforcement of temporal fidelity in
pyramidal cells by somatic feed-forward inhibition. Science 293, 1159–1163.
doi: 10.1126/science.1060342
Pravetoni, M., and Wickman, K. (2008). Behavioral characterization of mice
lacking GIRK/Kir3 channel subunits. Genes Brain Behav. 7, 523–531.
doi: 10.1111/j.1601-183x.2008.00388.x
Pressler, R. M., Boylan, G. B., Marlow, N., Blennow, M., Chiron, C., Cross, J. H.,
et al. (2015). Bumetanide for the treatment of seizures in newborn babies
with hypoxic ischaemic encephalopathy (NEMO): an open-label, dose finding
and feasibility phase 1/2 trial. Lancet Neurol. 14, 469–477. doi: 10.1016/s14744422(14)70303-5
Pueschel, S. M., Bernier, J. C., and Pezzullo, J. C. (1991). Behavioural observations
in children with Down’s syndrome. J. Ment. Defic. Res. 35, 502–511.
doi: 10.1111/j.1365-2788.1991.tb00447.x
Raccuglia, D., and Mueller, U. (2013). Focal uncaging of GABA reveals
a temporally defined role for GABAergic inhibition during appetitive
associative olfactory conditioning in honeybees. Learn. Mem. 20, 410–416.
doi: 10.1101/lm.030205.112
Ramamoorthi, K., and Lin, Y. (2011). The contribution of GABAergic
dysfunction to neurodevelopmental disorders. Trends Mol. Med. 17, 452–462.
doi: 10.1016/j.molmed.2011.03.003
Raz, N., Torres, I. J., Briggs, S. D., Spencer, W. D., Thornton, A. E., Loken, W. J.,
et al. (1995). Selective neuroanatomic abnormalities in Down’s syndrome and
their cognitive correlates: evidence from MRI morphometry. Neurology 45,
356–366. doi: 10.1212/WNL.45.2.356
Reefhuis, J., Devine, O., Friedman, J. M., Louik, C., Honein, M. A., and National
Birth Defects Prevention Study. (2015). Specific SSRIs and birth defects:
bayesian analysis to interpret new data in the context of previous reports. BMJ
351:h3190. doi: 10.1136/bmj.h3190
Reeves, R. H., Irving, N. G., Moran, T. H., Wohn, A., Kitt, C., Sisodia, S. S., et al.
(1995). A mouse model for Down syndrome exhibits learning and behaviour
deficits. Nat. Genet. 11, 177–184. doi: 10.1038/ng1095-177
Reinholdt, L. G., Czechanski, A., Kamdar, S., King, B. L., Sun, F., and Handel, M. A.
(2009). Meiotic behavior of aneuploid chromatin in mouse models of
Down syndrome. Chromosoma 118, 723–736. doi: 10.1007/s00412-0090230-8
Reynolds, G. P., and Warner, C. E. (1988). Amino acid neurotransmitter
deficits in adult Down’s syndrome brain tissue. Neurosci. Lett. 94, 224–227.
doi: 10.1016/0304-3940(88)90299-6
Rissman, R. A., and Mobley, W. C. (2011). Implications for treatment: GABAA
receptors in aging, Down syndrome and Alzheimer’s disease. J. Neurochem.
117, 613–622. doi: 10.1111/j.1471-4159.2011.07237.x
Rivera, C., Voipio, J., Payne, J. A., Ruusuvuori, E., Lahtinen, H., Lamsa, K., et al.
(1999). The K+ /Cl− co-transporter KCC2 renders GABA Hyperpolarizing
during neuronal maturation. Nature 397, 251–255. doi: 10.1038/16697
Robertson, J., Hatton, C., Emerson, E., and Baines, S. (2015). Prevalence of epilepsy
among people with intellectual disabilities: a systematic review. Seizure 29,
46–62. doi: 10.1016/j.seizure.2015.03.016
Roper, R. J., Baxter, L. L., Saran, N. G., Klinedinst, D. K., Beachy, P. A., and
Reeves, R. H. (2006). Defective cerebellar response to mitogenic Hedgehog
signaling in Down syndrome mice. Proc. Natl. Acad. Sci. U S A 103, 1452–1456.
doi: 10.1073/pnas.0601630103
Ross, M. H., Galaburda, A. M., and Kemper, T. L. (1984). Down’s syndrome:
is there a decreased population of neurons? Neurology 34, 909–916.
doi: 10.1212/WNL.34.7.909
Rost, B. R., Nicholson, P., Ahnert-Hilger, G., Rummel, A., Rosenmund, C.,
Breustedt, J., et al. (2011). Activation of metabotropic GABA receptors
increases the energy barrier for vesicle fusion. J. Cell Sci. 124, 3066–3073.
doi: 10.1242/jcs.074963
Roth, T., Roehrs, T., Wittig, R., and Zorick, F. (1984). Benzodiazepines and
memory. Br. J. Clin. Pharmacol. 18, 45S–49S. doi: 10.1111/j.1365-2125.1984.
tb02581.x
Ruby, N. F., Fernandez, F., Garrett, A., Klima, J., Zhang, P., Sapolsky, R.,
et al. (2013). Spatial memory and long-term object recognition are
Frontiers in Cellular Neuroscience | www.frontiersin.org
The GABAergic Hypothesis in Down Syndrome
impaired by circadian arrhythmia and restored by the GABAA antagonist
pentylenetetrazole. PLoS One 8:e72433. doi: 10.1371/journal.pone.0072433
Ruby, N. F., Fernandez, F., Zhang, P., Klima, J., Heller, H. C., and Garner, C. C.
(2010). Circadian locomotor rhythms are normal in Ts65Dn ‘‘Down
syndrome’’ mice and unaffected by pentylenetetrazole. J. Biol. Rhythms 25,
63–66. doi: 10.1177/0748730409356202
Ruby, N. F., Hwang, C. E., Wessells, C., Fernandez, F., Zhang, P., Sapolsky, R.,
et al. (2008). Hippocampal-dependent learning requires a functional circadian
system. Proc. Natl. Acad. Sci. U S A 105, 15593–15598. doi: 10.1073/pnas.
0808259105
Rudolph, U., and Knoflach, F. (2011). Beyond classical benzodiazepines: novel
therapeutic potential of GABAA receptor subtypes. Nat. Rev. Drug Discov. 10,
685–697. doi: 10.1038/nrd3502
Rueda, N., Flórez, J., and Martínez-Cué, C. (2008). Chronic pentylenetetrazole but
not donepezil treatment rescues spatial cognition in Ts65Dn mice, a model for
Down syndrome. Neurosci. Lett. 433, 22–27. doi: 10.1016/j.neulet.2007.12.039
Rueda, N., Flórez, J., and Martínez-Cué, C. (2012). Mouse models of down
syndrome as a tool to unravel the causes of mental disabilities. Neural Plast.
2012:584071. doi: 10.1155/2012/584071
Rueda, N., Llorens-Martín, M., Flórez, J., Valdizán, E., Banerjee, P., Trejo, J. L.,
et al. (2010). Memantine normalizes several phenotypic features in the
Ts65Dn mouse model of Down syndrome. J. Alzheimers Dis. 21, 277–290.
doi: 10.3233/JAD-2010-100240
Ruusuvuori, E., Huebner, A. K., Kirilkin, I., Yukin, A. Y., Blaesse, P.,
Helmy, M., et al. (2013). Neuronal carbonic anhydrase VII provides GABAergic
excitatory drive to exacerbate febrile seizures. EMBO J. 32, 2275–2286.
doi: 10.1038/emboj.2013.160
Sago, H., Carlson, E. J., Smith, D. J., Kilbridge, J., Rubin, E. M., Mobley, W. C.,
et al. (1998). Ts1Cje, a partial trisomy 16 mouse model for Down syndrome,
exhibits learning and behavioral abnormalities. Proc. Natl. Acad. Sci. U S A 95,
6256–6261. doi: 10.1073/pnas.95.11.6256
Sago, H., Carlson, E. J., Smith, D. J., Rubin, E. M., Crnic, L. S., Huang, T. T.,
et al. (2000). Genetic dissection of region associated with behavioral
abnormalities in mouse models for Down syndrome. Pediatr. Res. 48, 606–613.
doi: 10.1203/00006450-200011000-00009
Santin, M. D., Valabrèguea, R., Rivalsc, I., Pénagera, R., Paquina, R.,
Dauphinota, L., et al. (2015). In vivo 1 H MRS study in microlitre voxels in the
hippocampus of a mouse model of Down syndrome at 11.7 T. NMR Biomed.
27, 1143–1150. doi: 10.1002/nbm.3155
Scanziani, M. (2000). GABA spillover activates postsynaptic GABAB
receptors to control rhythmic hippocampal activity. Neuron 25, 673–681.
doi: 10.1016/s0896-6273(00)81069-7
Schmidt-Sidor, B., Wisniewski, K. E., Shepard, T. H., and Sersen, E. A. (1990).
Brain growth in Down syndrome subjects 15 to 22 weeks of gestational age and
birth to 60 months. Clin. Neuropathol. 9, 181–190.
Scott, F. F., Belle, M. D., Delagrange, P., and Piggins, H. D. (2010).
Electrophysiological effects of melatonin on mouse Per1 and non-Per1
suprachiasmatic nuclei neurones in vitro. J. Neuroendocrinol. 22, 1148–1156.
doi: 10.1111/j.1365-2826.2010.02063.x
Scott-McKean, J. J., Chang, B., Hurd, R. E., Nusinowitz, S., Schmidt, C.,
Davisson, M. T., et al. (2010). The mouse model of Down syndrome
Ts65Dn presents visual deficits as assessed by pattern visual evoked
potentials. Invest. Ophthalmol. Vis. Sci. 51, 3300–3308. doi: 10.1167/iovs.
09-4465
Scott-McKean, J. J., and Costa, A. C. (2011). Exaggerated NMDA mediated
LTD in a mouse model of Down syndrome and pharmacological rescuing by
memantine. Learn. Mem. 18, 774–778. doi: 10.1101/lm.024182.111
Seidl, R., Cairns, N., Singewald, N., Kaehler, S. T., and Lubec, G. (2001).
Differences between GABA levels in Alzheimer’s disease and Down syndrome
with Alzheimer-like neuropathology. Naunyn Schmiedebergs Arch. Pharmacol.
363, 139–145. doi: 10.1007/s002100000346
Shapiro, B. L. (2001). Developmental instability of the cerebellum and its relevance
to Down syndrome. J. Neural Transm. Suppl. 61, 11–34. doi: 10.1007/978-37091-6262-0_2
Siarey, R. J., Carlson, E. J., Epstein, C. J., Balbo, A., Rapoport, S. I., and Galdzicki, Z.
(1999). Increased synaptic depression in the Ts65Dn mouse, a model for
mental retardation in Down syndrome. Neuropharmacology 38, 1917–1920.
doi: 10.1016/s0028-3908(99)00083-0
19
March 2017 | Volume 11 | Article 54
Contestabile et al.
Siarey, R. J., Kline-Burgess, A., Cho, M., Balbo, A., Best, T. K., Harashima, C., et al.
(2006). Altered signaling pathways underlying abnormal hippocampal synaptic
plasticity in the Ts65Dn mouse model of Down syndrome. J. Neurochem. 98,
1266–1277. doi: 10.1111/j.1471-4159.2006.03971.x
Siarey, R. J., Stoll, J., Rapoport, S. I., and Galdzicki, Z. (1997). Altered long-term
potentiation in the young and old Ts65Dn mouse, a model for Down syndrome.
Neuropharmacology 36, 1549–1554. doi: 10.1016/s0028-3908(97)00157-3
Siarey, R. J., Villar, A. J., Epstein, C. J., and Galdzicki, Z. (2005). Abnormal synaptic
plasticity in the Ts1Cje segmental trisomy 16 mouse model of Down syndrome.
Neuropharmacology 49, 122–128. doi: 10.1016/j.neuropharm.2005.02.012
Silver, R. A. (2010). Neuronal arithmetic. Nat. Rev. Neurosci. 11, 474–489.
doi: 10.1038/nrn2864
Sim, H. R., Choi, T. Y., Lee, H. J., Kang, E. Y., Yoon, S., Han, P. L., et al. (2013). Role
of dopamine D2 receptors in plasticity of stress-induced addictive behaviours.
Nat. Commun. 4:1579. doi: 10.1038/ncomms2598
Śmigielska-Kuzia, J., Bockowski, L., Sobaniec, W., Kuak, W., and Sendrowski, K.
(2010). Amino acid metabolic processes in the temporal lobes assessed
by proton magnetic resonance spectroscopy (1H MRS) in children with
Down syndrome. Pharmacol. Rep. 62, 1070–1077. doi: 10.1016/s1734-1140(10)
70369-8
Śmigielska-Kuzia, J., and Sobaniec, W. (2007). Brain metabolic profile obtained
by proton magnetic resonance spectroscopy HMRS in children with Down
syndrome. Adv. Med. Sci. 52, 183–187.
Song, J., Zhong, C., Bonaguidi, M. A., Sun, G. J., Hsu, D., Gu, Y., et al. (2012).
Neuronal circuitry mechanism regulating adult quiescent neural stem-cell fate
decision. Nature 489, 150–154. doi: 10.1038/nature11306
Souchet, B., Guedj, F., Penke-Verdier, Z., Daubigney, F., Duchon, A.,
Herault, Y., et al. (2015). Pharmacological correction of excitation/inhibition
imbalance in Down syndrome mouse models. Front. Behav. Neurosci. 9:267.
doi: 10.3389/fnbeh.2015.00267
Stafstrom, C. E., Patxot, O. F., Gilmore, H. E., and Wisniewski, K. E. (1991).
Seizures in children with Down syndrome: etiology, characteristics and
outcome. Dev. Med. Child Neurol. 33, 191–200. doi: 10.1111/j.1469-8749.1991.
tb05108.x
Stagni, F., Magistretti, J., Guidi, S., Ciani, E., Mangano, C., Calzà, L., et al. (2013).
Pharmacotherapy with fluoxetine restores functional connectivity from the
dentate gyrus to field CA3 in the Ts65Dn mouse model of down syndrome.
PLoS One 8:e61689. doi: 10.1371/journal.pone.0061689
Staley, K. J., Soldo, B. L., and Proctor, W. R. (1995). Ionic mechanisms of
neuronal excitation by inhibitory GABAA receptors. Science 269, 977–981.
doi: 10.1126/science.7638623
Stasko, M. R., Scott-McKean, J. J., and Costa, A. C. (2006). Hypothermic responses
to 8-OH-DPAT in the Ts65Dn mouse model of Down syndrome. Neuroreport
17, 837–841. doi: 10.1097/01.WNR.0000220129.78726.bb
Stäubli, U., Scafidi, J., and Chun, D. (1999). GABAB receptor antagonism:
facilitatory effects on memory parallel those on LTP induced by TBS but not
HFS. J. Neurosci. 19, 4609–4615.
Steele, P. M., and Mauk, M. D. (1999). Inhibitory control of LTP and LTD: stability
of synapse strength. J. Neurophysiol. 81, 1559–1566.
Stern, S., Segal, M., and Moses, E. (2015). Involvement of potassium and cation
channels in hippocampal abnormalities of embryonic Ts65Dn and Tc1 trisomic
mice. EBioMedicine 2, 1048–1062. doi: 10.1016/j.ebiom.2015.07.038
Stewart, L. S., Persinger, M. A., Cortez, M. A., and Snead, O. C. III.
(2007). Chronobiometry of behavioral activity in the Ts65Dn model of
Down syndrome. Behav. Genet. 37, 388–398. doi: 10.1007/s10519-0069119-y
Succol, F., Fiumelli, H., Benfenati, F., Cancedda, L., and Barberis, A. (2012).
Intracellular chloride concentration influences the GABAA receptor subunit
composition. Nat. Commun. 3:738. doi: 10.1038/ncomms1744
Svirsky, N., Levy, S., and Avitsur, R. (2016). Prenatal exposure to selective
serotonin reuptake inhibitors (SSRI) increases aggression and modulates
maternal behavior in offspring mice. Dev. Psychobiol. 58, 71–82.
doi: 10.1002/dev.21356
Sylvester, P. E. (1983). The hippocampus in Down’s syndrome. J. Ment. Defic. Res.
27, 227–236. doi: 10.1111/j.1365-2788.1983.tb00294.x
Szabadics, J., Varga, C., Molnár, G., Oláh, S., Barzó, P., and Tamás, G. (2006).
Excitatory effect of GABAergic axo-axonic cells in cortical microcircuits.
Science 311, 233–235. doi: 10.1126/science.1121325
Frontiers in Cellular Neuroscience | www.frontiersin.org
The GABAergic Hypothesis in Down Syndrome
Szemes, M., Davies, R. L., Garden, C. L. P., and Usowicz, M. M. (2013). Weaker
control of the electrical properties of cerebellar granule cells by tonically active
GABAA receptors in the Ts65Dn mouse model of Down’s syndrome. Mol.
Brain 6:33. doi: 10.1186/1756-6606-6-33
Takahashi, K., Tanabe, K., Ohnuki, M., Narita, M., Ichisaka, T., Tomoda, K., et al.
(2007). Induction of pluripotent stem cells from adult human fibroblasts by
defined factors. Cell 131, 861–872. doi: 10.1016/j.cell.2007.11.019
Teipel, S. J., and Hampel, H. (2006). Neuroanatomy of down syndrome in vivo:
a model of preclinical Alzheimer’s disease. Behav. Genet. 36, 405–415.
doi: 10.1007/s10519-006-9047-x
Teipel, S. J., Schapiro, M. B., Alexander, G. E., Krasuski, J. S., Horwitz, B.,
Hoehne, C., et al. (2003). Relation of corpus callosum and hippocampal size
to age in nondemented adults with Down’s syndrome. Am. J. Psychiatry 160,
1870–1878. doi: 10.1176/appi.ajp.160.10.1870
Tyler, W. A., and Haydar, T. F. (2013). Multiplex genetic fate mapping
reveals a novel route of neocortical neurogenesis, which is altered in the
Ts65Dn mouse model of Down syndrome. J. Neurosci. 33, 5106–5119.
doi: 10.1523/JNEUROSCI.5380-12.2013
Usowicz, M. M., and Garden, C. L. (2012). Increased excitability and altered
action potential waveform in cerebellar granule neurons of the Ts65Dn mouse
model of Down syndrome. Brain Res. 1465, 10–17. doi: 10.1016/j.brainres.2012.
05.027
Velazquez, R., Ash, J. A., Powers, B. E., Kelley, C. M., Strawderman, M.,
Luscher, Z. I., et al. (2013). Maternal choline supplementation improves
spatial learning and adult hippocampal neurogenesis in the Ts65Dn mouse
model of Down syndrome. Neurobiol. Dis. 58, 92–101. doi: 10.1016/j.nbd.2013.
04.016
Venzi, M., Di Giovanni, G., and Crunelli, V. (2015). A critical evaluation of the
γ-hydroxybutyrate (GHB) model of absence seizures. CNS Neurosci. Ther. 21,
123–140. doi: 10.1111/cns.12337
Vicari, S. (2004). Memory development and intellectual disabilities. Acta Paediatr.
Suppl. 93, 60–63; discussion 63–64. doi: 10.1111/j.1651-2227.2004.tb03059.x
Vicari, S., Bellucci, S., and Carlesimo, G. A. (2000). Implicit and explicit memory: a
functional dissociation in persons with Down syndrome. Neuropsychologia 38,
240–251. doi: 10.1016/s0028-3932(99)00081-0
Vicari, S., Bellucci, S., and Carlesimo, G. A. (2005). Visual and spatial
long-term memory: differential pattern of impairments in Williams
and Down syndromes. Dev. Med. Child Neurol. 47, 305–311.
doi: 10.1017/s0012162205000599
Vicari, S., Pontillo, M., and Armando, M. (2013). Neurodevelopmental and
psychiatric issues in Down’s syndrome: assessment and intervention. Psychiatr.
Genet. 23, 95–107. doi: 10.1097/YPG.0b013e32835fe426
Vignes, M., Maurice, T., Lanté, F., Nedjar, M., Thethi, K., Guiramand, J., et al.
(2006). Anxiolytic properties of green tea polyphenol (-)-epigallocatechin
gallate (EGCG). Brain Res. 1110, 102–115. doi: 10.1016/j.brainres.2006.
06.062
Viitanen, T., Ruusuvuori, E., Kaila, K., and Voipio, J. (2010). The K+ Cl− cotransporter KCC2 promotes GABAergic excitation in the mature
rat hippocampus. J. Physiol. 588, 1527–1540. doi: 10.1113/jphysiol.2009.
181826
Wagner, S., Castel, M., Gainer, H., and Yarom, Y. (1997). GABA in the mammalian
suprachiasmatic nucleus and its role in diurnal rhythmicity. Nature 387,
598–603. doi: 10.1038/42468
Wang, X. J., and Buzsáki, G. (1996). γ oscillation by synaptic inhibition in a
hippocampal interneuronal network model. J. Neurosci. 16, 6402–6413.
Wang, D. D., and Kriegstein, A. R. (2009). Defining the role of GABA in
cortical development. J. Physiol. 587, 1873–1879. doi: 10.1113/jphysiol.2008.
167635
Wang, F., Li, J., Wu, C., Yang, J., Xu, F., and Zhao, Q. (2003). The GABAA receptor
mediates the hypnotic activity of melatonin in rats. Pharmacol. Biochem. Behav.
74, 573–578. doi: 10.1016/s0091-3057(02)01045-6
Waseem, T., Mukhtarov, M., Buldakova, S., Medina, I., and Bregestovski, P.
(2010). Genetically encoded Cl-Sensor as a tool for monitoring of Cl-dependent
processes in small neuronal compartments. J. Neurosci. Methods 193, 14–23.
doi: 10.1016/j.jneumeth.2010.08.002
Weick, J. P., Held, D. L., Bonadurer, G. F., Doers, M. E., Liu, Y., Maguire, C., et al.
(2013). Deficits in human trisomy 21 iPSCs and neurons. Proc. Natl. Acad. Sci.
U S A 110, 9962–9967. doi: 10.1073/pnas.1216575110
20
March 2017 | Volume 11 | Article 54
Contestabile et al.
Westmark, C. J., Westmark, P. R., and Malter, J. S. (2010). Alzheimer’s disease and
down syndrome rodent models exhibit audiogenic seizures. J. Alzheimers Dis.
20, 1009–1013. doi: 10.3233/JAD-2010-100087
Whitlock, J. R., Heynen, A. J., Shuler, M. G., and Bear, M. F. (2006). Learning
induces long-term potentiation in the hippocampus. Science 313, 1093–1097.
doi: 10.1126/science.1128134
Whittle, N., Sartori, S. B., Dierssen, M., Lubec, G., and Singewald, N. (2007). Fetal
Down syndrome brains exhibit aberrant levels of neurotransmitters critical
for normal brain development. Pediatrics 120, e1465–e1471. doi: 10.1542/peds.
2006-3448
Wigström, H., and Gustafsson, B. (1986). Postsynaptic control of hippocampal
long-term potentiation. J. Physiol. 81, 228–236.
Williams, J. T., Colmers, W. F., and Pan, Z. Z. (1988). Voltage- and ligandactivated inwardly rectifying currents in dorsal raphe neurons in vitro.
J. Neurosci. 8, 3499–3506.
Winter, T. C., Ostrovsky, A. A., Komarniski, C. A., and Uhrich, S. B. (2000).
Cerebellar and frontal lobe hypoplasia in fetuses with trisomy 21: usefulness
as combined US markers. Radiology 214, 533–538. doi: 10.1148/radiology.214.
2.r00fe40533
Wisden, W., Laurie, D. J., Monyer, H., and Seeburg, P. H. (1992). The distribution
of 13 GABAA receptor subunit mRNAs in the rat brain. I. Telencephalon,
diencephalon, mesencephalon. J. Neurosci. 12, 1040–1062.
Wisniewski, K. E. (1990). Down syndrome children often have brain with
maturation delay, retardation of growth and cortical dysgenesis. Am. J. Med.
Genet. Suppl. 7, 274–281. doi: 10.1002/ajmg.1320370755
Wisniewski, K. E., Dalton, A. J., McLachlan, C., Wen, G. Y., and Wisniewski, H. M.
(1985). Alzheimer’s disease in Down’s syndrome: clinicopathologic studies.
Neurology 35, 957–961. doi: 10.1212/WNL.35.7.957
Wisniewski, K. E., and Kida, E. (1994). Abnormal neurogenesis and
synaptogenesis in Down syndrome. Dev. Brain Dysfunction 7, 289–301.
Wisniewski, K. E., Laure-Kamionowska, M., and Wisniewski, H. M.
(1984). Evidence of arrest of neurogenesis and synaptogensis in brains
of patients with Down’s syndrome. N. Engl. J. Med. 311, 1187–1188.
doi: 10.1056/NEJM198411013111819
Wisniewski, K. E., and Schmidt-Sidor, B. (1989). Postnatal delay of myelin
formation in brains from Down syndrome infants and children. Clin.
Neuropathol. 8, 55–62.
Xie, W., Ramakrishna, N., Wieraszko, A., and Hwang, Y. W. (2008). Promotion
of neuronal plasticity by (−)-epigallocatechin-3-gallate. Neurochem. Res. 33,
776–783. doi: 10.1007/s11064-007-9494-7
Frontiers in Cellular Neuroscience | www.frontiersin.org
The GABAergic Hypothesis in Down Syndrome
Yu, T., Li, Z., Jia, Z., Clapcote, S. J., Liu, C., Li, S., et al. (2010a). A
mouse model of Down syndrome trisomic for all human chromosome
21 syntenic regions. Hum. Mol. Genet. 19, 2780–2791. doi: 10.1093/hmg/
ddQ199
Yu, T., Liu, C., Belichenko, P., Clapcote, S. J., Li, S., Pao, A., et al. (2010b). Effects
of individual segmental trisomies of human chromosome 21 syntenic regions
on hippocampal long-term potentiation and cognitive behaviors in mice. Brain
Res. 1366, 162–171. doi: 10.1016/j.brainres.2010.09.107
Zarrindast, M. R., Bakhsha, A., Rostami, P., and Shafaghi, B. (2002). Effects
of intrahippocampal injection of GABAergic drugs on memory retention
of passive avoidance learning in rats. J. Psychopharmacol. 16, 313–319.
doi: 10.1177/026988110201600405
Zhang, S. Y., Xu, M., Miao, Q. L., Poo, M. M., and Zhang, X. H. (2009).
Endocannabinoid-dependent homeostatic regulation of inhibitory synapses by
miniature excitatory synaptic activities. J. Neurosci. 29, 13222–13231. doi: 10.
1523/JNEUROSCI.1710-09.2009
Zhao, B., Zhu, J., Dai, D., Xing, J., He, J., Fu, Z., et al. (2016). Differential
dopaminergic regulation of inwardly rectifying potassium channel mediated
subthreshold dynamics in striatal medium spiny neurons. Neuropharmacology
107, 396–410. doi: 10.1016/j.neuropharm.2016.03.037
Zheleznova, N. N., Sedelnikova, A., and Weiss, D. S. (2009). Function and
modulation of δ-containing GABAA receptors. Psychoneuroendocrinology 34,
S67–S73. doi: 10.1016/j.psyneuen.2009.08.010
Conflict of Interest Statement: AC and LC are named as co-inventors on
International Patent Application PCT/EP2014/078561, filed on December 18,
2014, and connected US, EP, JP National Phase Applications, claiming priority to
US Provisional Application US 61/919,195, priority date December 20, 2013.
The other author SM declares that the research was conducted in the absence of
any commercial or financial relationships that could be construed as a potential
conflict of interest.
Copyright © 2017 Contestabile, Magara and Cancedda. This is an open-access
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provided the original author(s) or licensor are credited and that the original
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No use, distribution or reproduction is permitted which does not comply with these
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